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Property  of 

N.  C.  COLLEGE  OF  AGRICULTURE 

Department  of  Zoology  and  Entomology 


No. 


.wja^f**"**^*"'^ 


116182 


This  book  may  be  kept  out  TWO  WEEKS 
ONLY,  and  is  subject  to  a  fine  of  FIVE 
CENTS  a  day  thereafter.  It  is  due  on  the 
day  indicated  below: 


THE   GERM-CELL  CYCLE  IN  ANIMALS 


THE  MACMILLAN  COMPANY 

NEW  YORK    •    BOSTON    •    CHICAGO  -    DALLAS 
ATLANTA   •    SAN    FRANCISCO 

MACMILLAN  &  CO.,  Limited 

LONDON  •  BOMBAY  •  CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  Ltd. 

TORONTO 


THE  GERM-CELL  CYCLK 
IN  ANIMALS 


BY 


ROBERT  W.  HEGNER,  Ph.D. 

ASSIST.AJNT    PROFESSOR   OF   ZOOLOGY   IN   THE   UNIVERSTTY 

OF  IVnCHIGAN 


AUTHOR   OF    "an    IXTRODUCTIOX    TO    ZOOLOGY 
AND    "college    zoology" 


Neb)  gorfe 

THE   MACMILLAN   COjMPANY 

1914 

All  rights  reserved 


Copyright,  1914, 
By  the  MACMILLAN   COMPANY. 


Set  up  and  elcctrotyped.     Published  September,  1914. 


J.  S.  Gushing  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass,,  U.S.A. 


PREFACE 

This  book  is  the  result  of  a  course  of  lectures 
delivered  during  the  past  school  year  before  a  class 
in  Cellular  Biology  at  the  University  of  INIichigan. 
Many  of  the  most  important  recent  additions  to 
our  knowledge  of  heredity  have  resulted  from,  the 
study  of  the  germ  cells,  especially  those  of  animals. 
This  study  is  now  recognized  as  one  of  the  chief 
methods  of  attacking  certain  problems  in  genetics 
and  must  be  employed  in  correlation  with  animal 
breeding  before  we  can  hope  to  obtain  an  adequate 
explanation  of  the  results  of  hybridization.  For- 
tunately the  cytological  studies  of  the  germ  cells, 
both  observational  and  experimental,  have  kept 
pace  with  the  rapid  advances  in  our  knowledge  of 
plant  and  animal  breeding  which  have  been  made 
since  the  rediscovery  of  Mendel's  investigations  in 
1900.  The  term  "Germ-Cell  Cycle"  is  meant  to 
include  all  those  phenomena  concerned  with  the  ori- 
gin and  history  of  the  germ  cells  from  one  genera- 
tion to  the  next  generation.  The  writer  has,  with 
few  exceptions,  limited  himself  to  a  consideration  of 
the  germ  cells  in  animals  because  the  cycle  is  here 
more  definite  and  better  known  than  in  plants. 

It  is  obvious  to  any  one  familiar  with  this  subject 
that  only  a  few  of  the  many  interesting  phases  of 

1161S2 


vi  PREFACE 

the  problems  involved  can  be  considered  in  a  work 
of  this  size,  and  those  for  which  space  can  be  found 
must  be  limited  in  their  treatment.  For  this  reason 
some  periods  in  the  germ-cell  cycle  are  only  briefly 
mentioned,  whereas  others  are  more  fully  discussed. 
The  latter  are  naturally  those  in  which  the  writer 
is  most  interested  and  with  which  he  is  best  ac- 
quainted. Furthermore,  the  attempt  is  made  to 
present  the  data  available  in  such  a  way  as  to  make 
it  intelligible  to  those  who  have  not  been  able  to 
follow  in  detail  the  progress  of  cytology  during  the 
past  few  years.  This  can  only  be  accomplished  by 
introducing  many  facts  that  are  well  known  to 
cytologists  and  zoologists  in  general,  but  are  neces- 
sary for  the  presentation  of  a  complete  account  of 
the  subject. 

Much  of  the  recent  cytological  work  done  on  germ 
cells  has  emphasized  the  events  which  take  place 
during  the  maturation  of  the  eggs  and  spermatozoa, 
that  is,  the  periods  of  oogenesis  and  spermatogenesis. 
These  are,  of  course,  very  important  phases  of  the 
germ-cell  cycle,  but  they  should  not  be  allowed  to 
overshadow  the  rest  of  the  history  of  the  germ  cells. 
Contrary  to  the  usual  custom,  the  period  that  is 
emphasized  in  this  book  is  not  the  maturation  of 
the  germ  cells,  but  the  segregation  of  the  germ  cells 
in  the  developing  egg  and  the  visible  substances 
(keimbahn-determinants)  concerned  in  this  process. 

It  has  been  impossible  to  include  in  this  book  as 
much  illustrative  material  as  desirable,  but  the  bib- 
liography appended  indicates  what  data  exist  and 


PREFACE  vii 

where  they  may  be  obtained.  This  list  of  publica- 
tions has  been  arranged  according  to  the  method 
now  in  general  use  among  zoologists;  the  author's 
name  and  the  date  of  the  appearance  of  the  contri- 
bution in  question  are  bracketed  in  the  text  wher- 
ever it  has  been  considered  necessary,  and  reference 
to  the  list  at  the  end  of  the  book  will  reveal  the 
full  title  and  place  of  publication  of  the  work,  thus 
avoiding  cumbersome  footnotes.  The  figures  that 
have  been  copied  or  redrawn  are  likewise  referred 
in  every  case  to  the  original  source.  Many  of  them 
have  been  taken  from  the  writer's  previous  publica- 
tions and  a  few  have  been  made  especially  for  this 
work.  The  writer  has  likewise  drawn  freely  upon  the 
text  of  his  original  investigations  already  published. 

Ann  Arbor,  Michigan, 
April  16,  1914. 


TABLE  OF  CONTENTS 

CHAPTER  I 

PACK 

Introduction j 

The  Cell,  2 ;  Cell  Division,  13 ;  Methods  of  Repro- 
duction, 17;  The  Germ  Cells,  19;  The  Life  Cycles  of 
Animals,  22. 

CHAPTER  II 

General  Account  of  the   Germ-cell  Cycle  in  Ani- 
mals   C25 

Protozoa,  25 ;  Metazoa,  28. 

CHAPTER  III 

The    Germ-cell    Cycle    in    the    P^dogenetic    Fly, 

Miastor 51 

CHAPTER  IV 

The  Segregation  of  the  Germ  Cells  in  Sponges,  Cce- 

lenterates,  and  vertebrates        .         .         .       c9 

1.  Porifera,  69.  2.  Coelenterata,  80.  3.  Verte- 
brata,  98. 

CHAPTER  V 

The  Segregation  of  the  Germ  Cells  in  the  Artiiro- 

PODA lOG 

1.  The  Keimbahn  in  the  Insects,  106  ;  Diptera,  107 ; 
Coleoptera,  109  (In  Chrysomelid  Beetles,  109;  Origin 
of  Nurse  Cells,  119;  Cyst  Formation  in  Testis,  125; 
Amitosis,  133;  Differentiation  of  Nuclei  in  Egg,  141); 
Hymenoptera,  143.  2.  The  Keimbalui  in  tlie  Crusta- 
cea, 163. 

ix 


X  TABLE   OF   CONTENTS 

CHAPTER  VI 

PAGE 

The  Segregation  of  the  Germ  Cells  in  Nematodes, 

Sagitta,  and  Other  Metazoa       .         .        .     174i 

1.  The  Keimbahn  in  the  Nematodes,  174.  2.  The 
Keimbahn  in  Sagitta,  179.  3.  The  Keimbalin  in  Other 
Animals,  183. 

CHAPTER  Vn 

The  Germ  Cells  of  Hermaphroditic  Animals    .         .     189 

CHAPTER  VIII 

Keimbahn-Determinants  and  their  Significance       .     211 

A.  The  Genesis  of  the  Keimbahn-Determinants, 
211  (a.  Nuclear,  213  ;  b.  Cytoplasmic,  etc.,  224  ;  c.  Dis- 
cussion, 228) .  B.  The  Localization  of  the  Keimbahn- 
Determinants,  235.  C.  The  Fate  of  the  Keimbahn- 
Determinants,  240. 

CHAPTER  IX 

The  Chromosomes  and  Mitochondria  of  Germ  Cells    245 
The  Chromosome  Cycle  in  Animals,  245.    The  Mito- 
chondria of  Germ  Cells,  275. 

CHAPTER  X 

The  Germ-plasm  Theory 290 

References  to  Literature    .         .         .         .         .         .311 

Index  of  Authors  ......••     337 

Index  of  Subjects 341 


THE   GERM-CELL  CYCLE  IN  ANIMALS 


GERM-CELL  CYCLE  IN  ANIMALS 


CHAPTER  I 

INTRODUCTION 

Since  the  enunciation  by  Harvey  of  the  aphorism 
Omne  vivum  ex  ovo  in  the  seventeenth  century,  the 
statement  has  frequently  been  made  that  every 
animal  begins  its  individual  existence  as  an  egg. 
While  this  is  not  strictly  true,  since  no  eggs  occur  in 
the  life  history  of  many  one-celled  animals  (Pro- 
tozoa), and  a  large  number  of  multicellular  animals 
(Metazoa)  are  known  to  develop  from  buds  or  by 
fission,  still  the  majority  of  animals  arise  from  a  single 
cell  —  the  egg  (Fig.  4,  A) .  In  most  cases  this  egg, 
or  female  sex-cell,  is  unable  to  develop  in  nature 
unless  it  is  penetrated  by  a  spermatozoon  or  male 
sex-cell  (Fig.  4,  B).  The  single  cell  resulting  from  the 
fusion  of  an  egg  and  a  spermatozoon  is  known  as  a 
zygote.  One  of  the  most  remarkable  of  all  phenom- 
ena is  the  development  of  a  large,  com})lex  organisui 
from  a  minute,  and  apparently  simple,  zygote. 

According  to  the  older  scientists,  a  miniature  of  the 
adult  individual  was  present  in  the  egg,  and  (k*v(0- 
opment  consisted  in  the  growth  and   exi)ansion   of 

B  1 

D.  H.  HILL  LIBRARY 
North  Carolina  State  College 


2  GERM-CELL   CYCLE   IN  ANIMALS 

rudiments  already  preformed.  This  belief  could  not 
continue  to  exist  after  Caspar  Wolff's  brilliant 
researches  proved  that  adult  structures  arise  grad- 
ually from  apparently  undifferentiated  material ;  that 
is,  development  is  epigenetic.  Epigenesis,  however, 
does  not  explain  development;  it  simply  maintains 
that  it  occurs. 

During  the  years  since  the  theory  of  epigenesis 
was  proposed  a  new  theory  of  preformation  has 
entered  into  our  conception  of  development,  a  theory 
which  we  may  designate  as  predetermination.  We 
know  from  our  microscopical  studies  that  the  germ 
cells  possess  a  certain  amount  of  organization,  and 
that  the  zygote  contains  certain  structures  con- 
tributed by  the  egg  and  other  structures  brought  into 
the  egg  by  the  spermatozoon.  Hence,  to  a  certain 
extent,  development  is  predetermined,  since  the  initial 
structure  of  the  zygote  determines  the  characteristics 
of  the  individual  that  arises  from  it.  On  the  other 
hand,  development  is  also  epigenetic,  and  our  modern 
conception  includes  certain  features  of  each  theory. 

The  Cell.  A  brief  account  of  the  structure, 
physics,  and  chemistry  of  the  cell  will  serve  to  give  us 
some  idea  of  the  condition  of  the  zygote  from  which 
the  individual  arises,  and  will  help  us  to  understand 
certain  events  in  the  germ-cell  cycle  to  be  discussed 
later. 

The  cell  is  the  simplest  particle  of  matter  that  is 
able  to  maintain  itself  and  reproduce  others  of  its 
kind.  The  term  *ceir  was  applied  by  Hooke  in  1665 
to  the  cell-like  compartments  in  cork.     Cells  filled 


INTRODUCTION  3 

with  fluid  were  slightly  later  described  by  Malpighi. 
In  1833  Robert  Brown  discovered  nuclei  in  certain 
plant  cells.  What  is  known  now  as  the  Cell  Theory 
is  usually  dated  back  to  the  time  of  the  botanist 
Schleiden  (1838)  and  the  zoologist  Schwann  (1839), 
whose  investigations  of  the  cellular  phenomena  in 
animals  and  plants  added  greatly  to  the  knowledge 
of  these  units  of  structure.  At  this  time  the  cell- 
wall  was  considered  the  important  part  of  the  cell, 
but  continued  research  proved  this  idea  to  be  erro- 
neous. Schleiden  called  the  substance  within  the  cells 
plant  slime.  Later  (1846)  von  Mohl  gave  the  term 
protoplasm  to  the  same  substance.  The  substance 
within  the  animal  cell  was  named  sarcode  by  Du- 
jardin.  The  similarities  between  the  protoplasm  of 
plants  and  the  sarcode  of  animals  were  noted  by 
Cohn,  and  animal  cells  without  cell-walls  were 
observed  by  Kolliker  (1845).  It  was  not,  however, 
until  1861  that  Max  Schultze  finally  established  the 
fact  that  plant  protoplasm  and  animal  sarcode  are 
essentially  alike,  and  defined  the  cell  as  a  mass  of 
protoplasm  containing  a  nucleus.  Schultze's  re- 
searches serve  as  the  starting  point  for  modern 
studies  of  cellular  phenomena,  but  the  definition 
furnished  by  him  must  be  modified  slightly,  since  we 
now  know  that  many  cells  exist  without  definite 
nuclei.  These  cells,  however,  are  provided  with 
nuclear  material  scattered  throughout  the  cell  body 
(the  so-called  distributed  nucleus).  Our  definition 
must  be  changed  to  read,  a  cell  is  a  mass  of  proto- 
plasm  containing   nuclear   material.     Changes    like- 


4  GERM-CELL   CYCLE   IN  ANIMALS 

wise  have  taken  place  in  the  Cell  Theory;  we  no 
longer  consider  cells  as  isolated  units  and  the  multi- 
cellular animal  as  equivalent  to  the  sum  of  its  con- 
stituent cells,  but  recognize  the  influence  of  the  cells 
upon  one  another,  thus  reaching  the  conclusion  that 
the  metazoon  represents  the  sum  of  the  individual 
cells  plus  the  results  of  cellular  interaction. 

Cells  vary  considerably  in  size,  ranging  from  those 
we  call  Bacteria,  which  may  be  no  more  than  2  5  otto 
of  an  inch  in  length,  to  certain  egg  cells  which  are 
several  inches  long;  the  latter,  however,  owe  their 
enormous  size  to  the  accumulation  of  nutritive  sub- 
stances within  them.  An  average  cell  measures 
about  2  5V0  of  ^n  inch  in  diameter.  Cells  vary  in 
shape  as  well  as  in  size;  egg  cells  are  frequently 
spherical,  but  most  cells  are  not,  since  they  are  sur- 
rounded by  other  cells  which  press  against  them. 
A  diagram  of  a  typical  cell  is  shown  in  Fig.  1. 

Authorities  are  not  agreed  as  to  the  structure  of 
protoplasm;  to  some  it  appears,  as  shown  in  Fig.  1, 
to  consist  of  a  network  of  denser  fibers  called  spon- 
gioplasm  (s)  traversing  a  more  liquid  ground 
substance,  the  hyaloplasm.  Others  consider  proto- 
plasm to  be  alveolar  in  structure,  thus  resembling 
an  emulsion,  whereas  another  group  of  zoologists 
maintain  that  while  protoplasm  may  appear  to  be 
fibrillar  or  alveolar,  its  essential  basis  consists  of 
multitudes  of  minute  granules.  Wilson's  view  is 
the  one  usually  adopted  at  the  present  time ;  that 
is,  the  protoplasm  of  the  same  cell  may  pass  suc- 
cessively    "through     homogeneous,     alveolar,     and 


INTRODUCTION  5 

fibrillar  phases,  at  different  periods  of  growth  and 
in  different  conditions  of  physiological  activity," 
and  that  "apparently  homogeneous  protoplasm  is  a 
complex  mixture  of  substances  which  may  assume 


nm- 


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— m 


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^^G.  1.  —  Diagram  of  a  cell,  as  =  attraction-sphere;  c  =  centrosome; 
ch  =  chromatin  reticulum;  cr  =  chromidia;  ec  —  ectoplasm;  en  =  en- 
doplasm;  A:  =  karyosome;  Z  =  linin;  m  =  mitochondria;  we  =  meta- 
plasm;  nm  =  nuclear  membrane;  p  =  plastid;  pi  =  plasmosome  or 
nucleolus;  s  =  spongioplasm ;  v  =  vacuole. 

various  forms  of  visible  structure  according  to  its 
modes  of  activity." 

The  physical  properties  of  protoplasm  are  not  well 
known,  since  most  of  our  studies  have  been  made  with 
fixed  material.  We  know  that  protoplasm  may 
exist  as  a  gel  or  a  sol,  and  that  it  is  intermediate 
between  true  solids  and  true  liquids,  with  many  of 


6  GERM -CELL   CYCLE   IN  ANIMALS 

the  properties  of  each  and  a  number  of  properties 
pecuHar  to  itself.  No  doubt  the  protoplasm  differs 
in  its  physical  nature  in  different  cells.  In  the  egg 
of  the  starfish,  Asterias,  Kite  (1913)  has  shown  that 
the  cytoplasm  is  a  translucent  gel  of  comparatively 
high  viscosity  and  is  only  slightly  elastic ;  pieces 
become  spherical  when  separated  from  the  rest  of 
the  egg.  Scattered  throughout  this  gel  are  minute 
granules  (microsomes)  about  xoVo^  mm.  in  diameter 
which  cannot  be  entirely  freed  from  the  matrix. 
What  appear  to  be  alveoli  contain  globules  which 
possess  many  of  the  optical  properties  of  oil  drops; 
these  are  suspended  in  the  living  gel.  The  cyto- 
plasm of  the  starfish  egg  is  not  therefore  alveolar  in 
structure  as  usually  stated,  but  is  rather  of  the 
nature  of  a  suspension  of  microsomes  and  globules 
in  a  very  viscous  gel.  The  nuclear  membrane  is  a 
highly  translucent,  very  tough,  viscous  solid,  and 
not  a  delicate  structure  as  ordinarily  conceived. 
The  nucleolus  is  a  quite  rigid,  cohesive,  granular  gel 
suspended  in  the  sol  which  makes  up  the  rest  of  the 
nuclear  material.  Dividing  male  germ  cells  of  cer- 
tain insects  (squash  bugs,  grasshoppers,  and  crickets) 
revealed  the  fact  that  the  chromosomes  are  the  most 
highly  concentrated  and  rigid  part  of  the  nuclear 
gel ;  that  the  spindle  fibers  are  elastic,  concentrated 
threads  of  nuclear  gel ;  and  that  the  metaphase 
spindle  fibers  seem  to  be  continuous  with  the  ends  of 
the  chromosomes. 

The  ground  substance  of  the  nucleus  is  a  sol  termed 
nuclear  sap  or  karyolymph.     In  the  so-called  *rest- 


INTRODUCTION  7 

ing'  nucleus  a  network  of  fibers  may  be  observed 
similar  to  the  spongioplasm  in  the  cytoplasm ; 
these  consist  of  a  substance  named  linin  because  it 
usually  occurs  in  threads  (Fig.  1,  /).  Distributed 
along  the  linin  fibers  are  granules  of  a  substance  which 
stains  deeply  with  certain  dyes,  and  for  this  reason  is 
known  as  chromatin  (ch).  These  chromatin  gran- 
ules may  unite  to  form  larger  spherical  masses,  the 
karyosomes  or  chromatin-nucleoli  (A'),  and  during 
mitotic  nuclear  division  constitute  the  chromosomes 
(Fig.  3,  C).  In  many  cells  one  or  more  bodies 
resembhng  the  karyosomes  somewhat,  but  differing 
from  them  chemically  and  physiologically,  are  pres- 
ent; these  are  the  true  nucleoli  or  plasmosomes 
(Fig.  1,  pi).  Embedded  in  the  cytoplasm  near 
the  nucleus  may  often  be  seen  a  granular  body,  the 
centrosome  (c),  which  is  thought  to  be  of  great 
importance  during  mitotic  cell  division.  The  pro- 
toplasm surrounding  the  centrosome  is  usually  a 
differentiated  zone,  the  attraction-sphere  (as),  con- 
sisting of  archoplasm.  The  chromatin  whicli  may  be 
seen  in  the  cytoplasm  of  certain  cells  is  as  a  rule 
in  the  form  of  granules  called  chromidia  (cr).  Cer- 
tain other  cytoplasmic  inclusions  that  have  attractiMJ 
considerable  attention  within  the  past  fifteen  years 
exist  as  granules,  chains,  or  threads,  and  are  known  as 
mitochondria,  chondriosomes,  plastosomes,  etc.  (m). 
Various  sorts  of  plastids  (^),  such  as  chloroplastids 
and  amyloplastids,  may  be  present,  besides  a  varying 
number  of  solid  or  liquid  substances,  collectively 
designated  as  metaplasm  (me)  or  para])lasni,  which 


8  GERM-CELL   CYCLE  IN  ANIMALS 

are  not  supposed  to  form  part  of  the  living  sub- 
stance ;  these  are  pigment  granules,  fat  globules, 
excretory  products,  vacuoles  (y),  etc. 

It  has  been  found  possible  to  explain  many  cellular 
activities  and  even  the  results  obtained  by  experi- 
mental animal  breeding  by  studies  of  the  physics 
and  chemistry  of  protoplasm.  An  exhaustive  ac- 
count of  the  subject  is  impossible  and  even  unneces- 
sary here,  but  the  importance  assigned  to  the  physico- 
chemical  explanation  of  life  phenomena  requires  a 
brief  statement.  Kossel  has  separated  the  cellular 
constituents  into  two  main  groups.  (1)  Primary 
constituents  are  those  necessary  for  life ;  these  are 
water,  certain  minerals,  proteins,  nucleoproteins, 
phosphatides  (lecithin),  cholesterin,  and  perhaps 
others.  (2)  Secondary  constituents  are  not  essen- 
tially necessary  and  do  not  occur  in  every  cell ; 
they  are  usually  stored  up  reserve  material  or  meta- 
bolic products  representing  principally  what  we  have 
termed  metaplasm. 

Water  which  constitutes  about  two-thirds  of  the 
animal  is  necessary  for  the  solution  of  various  bodies, 
the  dissociation  of  chemical  compounds,  the  exchange 
of  materials,  the  removal  of  metabolic  products, 
etc.  Mineral  substances  are  present  in  all  animal 
tissues,  and  different  tissues  are  characterized  by 
the  presence  of  different  minerals.  The  principal 
ones  are  potassium,  sodium,  calcium,  magnesium, 
iron,  phosphoric  acid,  sulphuric  acid,  and  chlorine. 
The  other  constituents  are  of  a  colloidal  nature, 
and  its  richness  in  colloids  is  one  of  the  chief  charac- 


INTRODUCTION  9 

teristics  of  protoplasm.  To  understand  the  activi- 
ties of  protoplasm  we  must  therefore  know  sometliiiig 
of  the  physics  and  chemistry  of  colloids. 

Colloids  (from  colla  =  glue)  do  not  diffuse,  or 
diffuse  very  slowly,  through  animal  membranes ;  in 
this  respect  they  differ  from  crystalloids,  which 
diffuse  comparatively  rapidly  through  animal  mem- 
branes. Wolfgang  Ostwald  recognized  two  sorts 
of  colloids  :  (1)  suspension  colloids,  which  are  mix- 
tures of  solid  and  liquid  phases,  are  non-viscous, 
and  easily  coagulated  by  salts,  e.g.  a  mixture  of 
finely  divided  metal  and  water ;  and  (2)  emulsion 
colloids,  which  are  composed  of  two  liquid  phases, 
are  viscous,  and  coagulated  by  salts  with  difficulty. 
Protoplasm  is  rich  in  emulsion  colloids ;  these  may 
exist  as  liquid  sols,  or  more  solid  gels.  In  either 
case  they  consist  of  fine  colloidal  particles.  Accord- 
ing to  another  classification  colloids  may  be  separated 
into  reversible  and  irreversible ;  the  former  may 
change  from  the  sol  to  the  gel  state  and  back  again, 
but  the  latter  are  unable  to  do  this.  Protoplasm  is 
a  reversible  colloid,  and  many  cellular  structures 
appear  to  originate  through  the  gelation  of  licjuid 
colloids.  Since  protoplasm  is  a  sol  or  gel  due  to 
water,  it  is  a  hydrosol  or  hydrogel,  and  because 
of  its  water  content  is  said  to  be  hydrophylic.  Il 
'  contains  crystalloids  and  its  chemical  reactions  take 
place  in  a  dilute  solution  of  electrolytes ;  these  are 
substances  which  dissociate,  at  least  in  part,  into 
their  constituent  ions  when  in  solution,  and  the  ions 
are  electrically  charged.     For  example,  NaCl  disso- 


10  GERM-CELL  CYCLE   IN  ANIMALS 

dates  into  electro-positive  Na  ions  (cations)  and 
electro-negative  CI  ions  (anions).  Colloidal  par- 
ticles are  likewise  electrically  charged,  those  of  acid 
colloids  usually  negatively  and  those  of  alkaline 
colloids  positively.  The  union  and  separation  of 
particles  and  their  consequent  rearrangement  cause 
gelation,  liquefaction,  etc. ;  it  is  thus  evident  that 
many  physiological  activities  may  be  due  to  the 
electrical  charges  of  ions  instead  of  the  chemical 
nature  of  the  particles  themselves.  Cellular  struc- 
tures therefore  depend  upon  the  tendency  of  col- 
loidal particles  to  form  aggregates  (gelation,  coagula- 
tion), and  more  or  less  upon  the  electrically  charged 
nature  of  the  particles. 

The  most  characteristic  chemical  constituents  of 
protoplasm  are  the  proteins.  The  most  common 
proteins  in  the  body  show  on  the  average  the  follow- 
ing percentage  of  elements  :  — 

Carbon 50    -55    % 

Hydrogen 6.5-  7.3% 

Nitrogen 15    -17.6% 

Oxygen 19    -24    % 

Sulphur 3-  2.4% 

Proteins  may  be  separated  into  three  groups  :  (1) 
simple  proteins,  such  as  protamines,  albumins,  and 
globulins;  (2)  conjugated  proteins,  the  glucopro- 
teins,  nucleoproteins,  and  chromoproteins ;  and 
(3)  the  products  of  protein  hydrolysis,  infraproteins, 
proteoses,  peptones,  and  polypeptides.  These  have 
been  studied  both  by  microchemical  and  macro- 
chemical  methods.  In  the  former  method  reagents 
are    applied    to    the    microscopic    objects    and    the 


INTRODUCTION  11 

changes  in  color,  etc.,  indicate  its  constitution ;  e.g., 
iron  and  phosphorus  may  be  detected  in  this  way. 
Parts  showing  affinity  for  acid  stains  Hke  eosin  are 
said  to  be  acidophile  or  oxyphile ;  those  showing 
affinity  for  basic  dyes,  Hke  methylene  blue,  are 
called  basophile.  The  chromatin  is  basophile, 
whereas  the  linin  and  cytoplasm  are  oxyphile.  In 
macrochemistry  large  quantities  of  the  substances 
are  collected  and  examined  by  ordinary  laboratory 
methods. 

Because  of  the  importance  that  has  been  assigned 
to  the  chromatin,  this  substance  is  particularly 
interesting.  Chromatin  consists  of  nuclein,  which  is 
a  conjugated  protein  containing  nucleic  acid,  the 
latter  being  an  organic  acid,  rich  in  phosphorus ; 
it  is  hence  called  nucleoprotein.  Nucleoproteins 
are  found  chiefly  in  the  nucleus  but  also  occur  in 
the  cytoplasm.  They  may  differ  from  one  another 
in  their  protein  content  as  well  as  in  the  character 
of  their  nucleic  acid  constituent.  When  treated 
with  dilute  acids  nuclein  is  obtained,  and  when  this 
is  further  subjugated  to  caustic  alkali  it  decomposes 
into  protein  and  nucleic  acid.  The  nucleic  acids 
w^hich  have  been  principally  studied  are  those  de- 
rived from  the  thymus  gland,  and  from  the  si)erma- 
tozoa  of  salmon,  herring,  and  other  fish  ;  they  are 
probably  all  the  same.  Levene  (1910)  recognizes 
three  sorts  of  nucleic  acid,  of  which  the  most  complex 
is    termed    thymonucleic    acid.     This    consists    of 

two  purine  bases,  guanine  and  adenine ; 

two  pyrimidine  bases,  thymine  and  cytosine ; 


12  GERM-CELL   CYCLE  IN  ANIMALS 

a  hextose  (carbohydrate) ;   and 

phosphoric  acid. 
Its  formula,  according  to  Schmiedeberg,  is  C40H56 
N14O16 . 2  P2O5,  and  according  to  Steudel,  C43H57 
N15O12 . 2  P2O5.  Considerable  progress  has  been 
made,  especially  by  Emil  Fischer  and  his  students, 
in  the  synthesis  of  protein-like  bodies.  Many  com- 
plex polypeptides  have  been  built  up  which  resemble 
peptones  in  many  of  their  reactions  and  when  in- 
jected into  living  organisms  appear  to  be  utilized 
in  metabolism  in  much  the  same  way  as  are  native 
proteins. 

We  are  still,  however,  very  far  from  an  adequate 
understanding  of  the  nature  of  chromatin.  Delia 
Valle  (1912),  for  example,  after  an  exhaustive  study 
of  the  physico-chemical  properties  of  chromatin 
both  in  the  resting  nucleus  and  in  the  dividing  cell, 
has  concluded  that  this  substance  resembles  that 
of  fluid  crystals.  "Consequently  all  of  the  pheno- 
mena presented  by  the  chromosomes  ;  their  mode  of 
origin,  differences  in  size,  state  of  aggregation,  form, 
structure,  colorability,  optical  characteristics,  varia- 
tions in  form,  longitudinal  division  and  the  phenom- 
ena which  follow  this  mode  of  scattering,  demon- 
strate that  the  chromosomes  are  crystalloids." 

Two  other  primary  constituents  of  protoplasm  may 
be  mentioned  briefly.  The  phosphatide,  lecithin, 
belongs  with  cholesterin  to  a  group  of  compounds 
called  lipoids.  It  consists  of  glycerophosphoric  acid 
plus  certain  fatty  acid  radicles,  such  as  stearic  acid, 
oleic  acid,  etc.,  and  a  nitrogenous  base  (cholin).     It 


INTRODUCTION  13 

probably  plays  some  part  in  cell  metabolism,  may 
furnish  material  for  building  up  nucleins,  and  ac- 
cording to  Faure-Fremiet  is  concerned  in  the  forma- 
tion of  mitochondria.  Cholesterin  is  considered  a 
waste  product  of  cell  life,  although  it  is  known  to  in- 
hibit haemolysis  produced  by  certain  bodies  and  is 
thus  a  protective  against  toxins,  and  may  have 
other  functions.  We  should  look  forward  with  great 
interest  to  the  results  of  investigations  that  are  now 
being  carried  on  by  biochemists,  since  we  depend 
upon  them  for  an  explanation  of  many  of  the  phe- 
nomena of  life,  cellular  differentiation,  and  heredity. 
We  even  hope  that  they  may  be  able  to  create 
compounds  in  the  laboratory  that  we  may  consider 
living  organisms.  However,  the  task  does  not 
seem  to  be  so  simple  to  the  biochemist,  who  should 
know,  as  it  does  to  the  biologist.  Nevertheless, 
as  Jacques  Loeb  has  said,  we  should  "either  succeed 
in  producing  living  matter  artificially,  or  find  the 
reasons  why  this  should  be  impossible." 

Cell  Division.  Cells  may  increase  in  number  by 
direct  (amitotic)  or  indirect  (mitotic  or  karyokinetic) 
division.  There  is  no  doubt  that  mitosis  occurs, 
but  not  all  investigators  are  convinced  that  cells 
ever  divide  amitotically.  Direct  division  was  once 
considered  the  only  method  of  cell  nudtiplication.  It 
was  described  as  a  simple  division  of  the  nucleus 
into  two  parts  (Fig.  2),  preceded  by  a  division  of  the 
nucleolus  into  two,  and  succeeded  by  a  constriction 
of  the  entire  cell ;  the  result  was  two  daughter  cells 
each  with  one  nucleus  containing   one-half  of  the 


14 


GERM-CELL   CYCLE   IN   ANIMALS 


nucleolus .  As  we  shall  see  later  (Chapter  V) ,  amitosis 
has  been  described  in  cells  of  the  germ-cell  cycle, 
and  must  therefore  be  reckoned  with  in  any  discus- 
sion of  the  phys- 
ical  basis  of 
heredity. 

Mitosis  or  ka- 
ryokinesis  in- 
volves a  rather 
complicated 
series  of  pro- 
cesses which 
cannot  be  fully 
discussed  here 
but  will  be  out- 
lined very  briefly 
with  the  aid  of 
Fig.  3. 

(a)  During 
the  prophase 
the  chromatin 
granules  which 
are  scattered 
through  the  nucleus  in  the  resting  cell  (A)  become 
arranged  in  the  form  of  a  long  thread  or  spireme  (B) . 
At  the  same  time  the  centrosomes  move  apart  (A,  c; 
B,  a),  and  a  spindle  arises  between  them  (C).  While 
this  is  going  on,  the  nuclear  membrane  generally 
disintegrates  and  the  spireme  segments  into  a  num- 
ber of  bodies  called  chromosomes  (C) ;  these  take  a 
position  at  the  equator  of  the  spindle,  halfway  be- 


Fig.  2.  —  Amitosis.  A.  Division  of  blood-cells 
in  the  embryo  chick,  illustrating  Remak's 
scheme,  a—e  =  successive  stages  of  division. 
{From  Wilson,  1900.)  B.  Amitotic  nuclear 
division  in  the  follicle  cells  of  a  cricket's  egg. 
{From  Dahlgren  and  Kepner,  1908.) 


INTRODUCTION 


15 


tween  the  centrosomes  (Z),  ep).     The  stage  shown  in 
Fig.  3,  J9,  is  known  as  the  amphiaster ;  at  this  time 


D 


B 


E 


Fig.  3.  —  Mitosis.  Diagrams  illustrating  mitotic  cell  division.  {From 
Wilson.)  A,  resting  cell;  B,  prophase  showing  spireme  and  nucle- 
olus within  the  nucleus  and  the  formation  of  spindle  and  asters  (n)', 
C,  later  prophase  showing  disintegration  of  nuclear  memhrane,  and 
breaking  up  of  spireme  into  chromosomes;  D,  end  of  prophases, 
showing  complete  spindle  and  asters  with  chromosomes  in  etiuatorial 
plate  (ep);  E,  metaphase  —  each  chromosome  splits  in  two;  F,  ana- 
phase —  the  chromosomes  are  drawn  toward  the  asters,  i'/=  inter- 
zonal fibers;  G,  telophase,  showing  reconstruction  of  nuclei;  //.latex 
telophase,  showing  division  of  the  cell  into  two. 

all  of  the  mechanism  concerned  in  mitosis  is  present. 
There  are  two  asters,  each  consisting  of  a  centrosome 


16  GERM-CELL   CYCLE   IN  ANIMALS 

surrounded  by  a  number  of  radiating  astral  rays,  and 
a  spindle  which  lies  between  them.  The  chromo- 
somes lie  in  the  equatorial  plate  [ej)). 

(b)  During  the  second  stage,  the  metaphase,  the 
chromosomes  split  in  such  a  way  that  each  of  their 
parts  contains  an  equal  amount  of  chromatin  {E,  ep) . 
As  we  shall  see  later,  this  is  one  of  the  most  significant 
events  that  takes  place  during  mitosis. 

(c)  During  the  anaphase  (F)  the  chromosomes 
formed  by  splitting  move  along  the  spindle  fibers 
to  the  centrosomes.  As  a  result  every  chromosome 
present  at  the  end  of  the  prophase  (D)  sends  half  of  its 
chromatin  to  either  end  of  the  spindle.  The  mechan- 
ism that  brings  about  this  migration  is  as  yet  some- 
what in  question.  Fibers  are  usually  left  between 
the  separating  chromosomes;  these  are  known  as 
interzonal  fibers  (F,  if). 

(d)  The  telophase  (G,  H)  is  a  stage  of  reconstruction 
from  which  the  nuclei  emerge  in  a  resting  condition ; 
the  chromatin  becomes  scattered  through  the  nucleus, 
which  is  again  enveloped  by  a  definite  membrane 
(H) ;  the  centrosome  divides  and,  with  the  centro- 
sphere,  takes  a  position  near  the  nucleus.  Finally 
the  cycle  is  completed  by  the  constriction  of  the  cell 
into  two  daughter  cells. 

There  are  a  number  of  differences  between  the 
sort  of  mitosis  just  described  and  that  which  occurs 
during  the  maturation  of  the  egg  and  spermatozoon ; 
these  and  certain  other  phases  of  cell  division  will 
be  considered  in  their  appropriate  places  in  succeed- 
ing chapters. 


INTRODUCTION  17 

Methods  of  Reproduction.  In  the  beginning 
paragraph  of  this  chapter  it  was  stated,  with  reserva- 
tions, that  every  individual  develops  from  an  v^^. 
Before  we  can  discuss  the  germ-cell  cycle  intelh- 
gently,  however,  we  must  consider  the  exceptions 
to  this  rule,  and  outline  as  briefly  as  possible  the 
various  methods  of  reproduction  which  are  known 
to  occur  among  animals.  Reproduction  is  the  forma- 
tion of  new  individuals  by  division  ;  this  is  frequently 
preceded  by  conjugation  (in  the  Protozoa)  or  fertil- 
ization (in  both  the  Protozoa  and  the  Metazoa). 

Three  principal  methods  of  reproduction  occur 
in  the  Protozoa.  (1)  Binary  fission  appears  to 
be  the  most  primitive.  The  individual  divides  into 
two  parts  which  are  similar  in  size  and  structure ; 
these  grow  into  cells  like  the  original  parent.  Many 
CiLiATA,  Flagellata,  and  Rhizopoda  normally 
reproduce  in  this  way.  (2)  Budding  occurs  when 
a  small  outgrowth  or  bud  separates  from  the  parent 
cell.  This  method  occurs  among  the  Suctoria, 
Radiolaria,  Heliozoa,  Ciliata,  and  Myxospo- 
ridia.  (3)  Sporulation  results  from  the  division  of 
the  nucleus  of  the  parent  into  many  daughter  nuclei 
and  a  subsequent  division  of  the  cell  into  as  many 
"spores"  as  there  are  nuclei.  This  process  is 
characteristic  of  the  Sporozoa  and  also  is  found 
among  the  Rhizopoda.  Conjugation  is  of  freqnent 
occurrence  in  the  Protozoa.  Two  or  more  inch*- 
viduals  may  become  connected  without  fusion  of 
nuclei  or  cytoplasm,  thus  forming  colonies  :  a  pair  of 
individuals   may   unite   either   temporarily    or   pvr- 


18  GERM-CELL  CYCLE  IN  ANIMALS 

manently  with  fusion  of  the  cytoplasm  only;  or 
both  cytoplasm  and  nuclei  of  such  a  pair  may  fuse 
or  be  interchanged. 

Metazoa  reproduce  either  sexually  or  asexually. 
Asexual  reproduction  is  reproduction  without  the  aid 
of  sex  cells.  It  takes  place  as  a  rule  by  means  of 
buds  or  by  fission  as  in  many  polyps,  sponges, 
flat- worms,  segmented  round-worms,  and  bryozoans. 
Even  the  tunicates,  which  occupy  an  advanced  posi- 
tion in  the  animal  series,  form  buds.  Some  of  the 
sponges  produce  internal  buds  called  gemmules, 
and  certain  bryozoans  form  similar  bodies  known 
as  statoblasts.  Sexual  reproduction  requires  that 
the  individual  develop  from  a  mature  egg.  As  a  rule 
the  egg  must  be  fertilized  by  the  union  with  it  of  a 
spermatozoon,  thus  forming  a  zygote ;  but  the  eggs 
.of  many  animals  develop  without  being  fertihzed ; 
that  is,  they  are  parthenogenetic.  In  rare  cases  such 
parthenogenetic  eggs  may  be  produced,  as  in  the 
fly  Miastor  (see  Chapter  III),  by  immature  individ- 
uals. When  this  occurs,  reproduction  is  said  to  be 
poedogenetic. 

The  sex  of  an  animal  is  judged  by  the  kind  of  sex 
cells  it  produces,  —  eggs  by  the  female  and  sperma- 
tozoa by  the  male,  —  and  when  the  individuals  of  a 
single  species  are  differentiated  as  either  males  or 
females,  the  species  is  said  to  be  dioecious  and  the 
individuals  gonochoristic.  In  many  species  there  is 
but  a  single  sort  of  individual  which  produces  both 
eggs  and  spermatozoa ;  such  species  are  monoecious, 
and  the  individuals  are  hermaphroditic. 


INTRODUCTION  W 

The  Germ  Cells.  Eggs  and  spermatozoa  differ 
from  each  other  both  morphologically  and  physiolog- 
ically. Eggs  are  usually  spherical  or  oval  in  shape 
(Fig.  4),  although  they  may  vary  greatly  from  the 
typical  form  and  may  even  be  ameboid  as  in  certain 
coelenterates.  In  size  they  range  from  that  of  tlie 
mouse,  which  is  only  about  0.065  mm.  in  diameter,  to 
that  of  birds,  which  are  several  inches  long.  The 
large  volume  of  the  latter  is  due  to  the  presence  of 
an  enormous  amount  of  nutritive  material,  and  the 
general  statement  may  be  made  that  the  size  of  an 
egg  does  not  depend  so  much  upon  the  size  of  the 
animal  as  upon  the  amount  of  yolk  stored  within  it. 
The  egg  nucleus,  which  is  frequently  very  large  and 
clear,  is  known  as  the  germinal  vesicle ;  and  its 
nucleolus  has  often  been  referred  to  as  the  germinal 
spot.  Embedded  within  the  cytoplasm  of  the  ovum 
are  several  bodies  besides  the  yolk  globules.  A 
"yolk  nucleus"  may  be  present;  mitochondrial 
granules  or  rods  may  occur ;  and  special  inclusions, 
which  become  associated  with  the  primordial  germ 
cells  and  have  been  named  keimbahn-determinants, 
have  been  recorded  in  many  cases.  Considerable 
evidence  has  accumulated  that  the  egg  substance 
is  not  a  homogeneous,  isotropic  mixture,  but  is  def- 
initely organized,  and  that  this  organization  is 
related  to  the  morphology  of  the  embryo  which  is 
to  develop  from  it;  hence  we  speak  of  the  promor- 
phology  of  the  egg.  Eggs  are  said  to  possess  polarity, 
and  even  the  oogonium  as  it  lies  in  the  ovary  is 
definitely   oriented   with   respect   to   its   chief  axes. 


20 


GERM- CELL  CYCLE  IN  ANIMALS 


The  principal  poles  are  dissimilar ;  the  end  of  the  egg 
containing  most  of  the  cytoplasm  and  nearer  which 
lie  the  nucleus  and  centrosome  is  known  as  the 
animal  pole ;    the  other  end,  which  is  often  crowded 


cm* 


Fig.  4.  —  Germ  cells.  Ovarian  ovum  of  a  cat  just  before  maturity. 
0.771.=  cell  membrane;  mics.  =  microsomes;  ncl  =  nucleolus;  n.  m  = 
nuclear  membrane;  y/c.  aZ.  =  yolk  alveoli.  {From  Dahlgren  and 
Kepner.) 

with  the  yolk  globules,  is  called  the  vegetative  pole. 
The  subject  of  the  organization  of  the  egg  will  be 
referred  to  more  in  detail  later  (Chapter  VIII). 

The    male    sex   cells   or   spermatozoa   differ    very 
strikingly  from  the  eggs.     They  are  usually  of  the 


INTRODUCTION 


21 


'  Apical  body  or  acrosome. 


Nucleus. 


•  End-knob. 

•  Middle-piece. 


•  Envelope  of  the  tail. 


-Axial  filament. 


flagellate  type  (Fig.  4a),  consisting  of  a  head,  largely 
made  up  of  chromatin,  a  middle  piece,  and  a  vibratile 
tail.     Spermatozoa  are  comparatively  minute,  rang- 
ing in  size  from  those  of  Amphioxiis,  which  are  less 
than  0.02  mm.  long,  to  those  of 
the      amphibian,      Discoglossus, 
which  reach  a  length  of  2.0  mm. 
According   to  Wilson    it  would 
take    from    400,000    to    500,000 
sea  urchin  spermatozoa  to  equal 
in  volume  the  egg  of  the  same 
species.      It   is    not    surprising, 
therefore,  to  find  that  the  num- 
ber of  spermatozoa  produced  by 
a  single  male  may  be  hundreds 
of  thousands  times   as  great  as 
the  number  of  eggs  developed 
in  a  female.     Eggs  are,  as  a  rule, 
incapable     of    locomotion,    but 
spermatozoa    are   active,   swim- 
ming about   by  means  of  their 
tails  until  they  reach  the  passive 
eggs  which  they  are  to  fertilize. 
Since  generally  only  one  sperm- 
atozoon fuses  with  an  egg,  it  is 
obvious  that  most  of  them  never  perform  the  function 
for  which  they  are  specialized ;   but  apparently  an 
enormous  number  are  formed  to  make  the  ferliHza- 
tion  of  the  eggs  more  certain. 

The  experiments  of  Loeb  and  Bancroft  (1912)  on 
spermatozoa    have    shown    that    when    the    living 


.  End-piece. 


Fig.  -ia.  —  Diagram  of  a 
flagellate  spermatozoon. 
{From  WUson,  1900.) 


22  GERM-CELL   CYCLE   IN  ANIMALS 

spermatozoa  of  the  fowl  are  placed  in  a  hanging  drop 
of  white  of  egg  or  in  yolk  they  undergo  a  transfor- 
mation into  nuclei.  The  possibility  that  a  sperma- 
tozoon may  give  rise  to  an  embryo  without  the  help 
of  an  egg  is  recognized,  but  this  has  not  yet  been 
accomplished. 

The  Life  Cycles  of  Animals.  The  life  cycle 
of  an  animal  has  considerable  influence  upon  the 
course  of  the  germ-cell  cycle.  In  all  animals  that 
are  produced  by  the  sexual  method  the  beginning 
stage  in  the  life  cycle  is  a  mature  egg,  either  fertilized 
or  unfertilized  according  to  the  species.  Animals 
which  develop  asexually,  on  the  other  hand,  begin 
their  cycle  with  the  first  recognizable  evidence  of 
budding  or  fission.  As  a  rule  budding  or  fission  are 
sooner  or  later  interrupted  by  the  formation  of  sex 
cells,  hence  the  life  cycle  of  such  animals  may  be 
considered  to  extend  from  the  mature  egg  to  that 
stage  in  the  life  history  of  the  species  when  mature 
eggs  are  again  produced.  Such  a  life  cycle  consists 
really  of  two  or  more  simple  life  cycles  represented 
by  individuals  differing  from  one  another  in  both 
structure  and  method  of  reproduction.  As  examples 
of  some  of  the  principal  types  of  life  cycles  we  may 
select  certain  insects  and  coelenterates. 

A  very  simple  life  cycle  is  that  of  the  wingless 
insects  of  the  order  After  a.  The  young,  when  they 
hatch  from  the  egg,  are  similar  in  form,  structure, 
and  habits  to  the  fully  grown  individual  and  undergo 
no  perceptible  changes,  except  increase  in  size, 
until    they    become    sexually    mature    adults.     In 


INTRODUCTION  23 

certain  other  groups  of  insects,  such  as  the  grass- 
hoppers, the  newly  hatched  young  resemble  the 
adult  in  many  ways,  differing  i)rincipally  in  the 
absence  of  wings.  The  young  Rocky  Mountain 
locust  (Melanoplus  spretus),  for  example,  changes 
its  exoskeleton  (molts)  five  times  before  the  adult 
condition  is  attained.  After  each  molt  there  are 
slight  changes  in  color,  structure,  and  size,  the  most 
notable  difference  being  the  gradual  acquirement  of 
wings.  In  still  other  orders  of  insects  a  larva 
hatches  from  the  egg ;  this  larva,  on  reaching  its  full 
growth,  changes  in  shape  and  structure,  becoming  a 
quiescent  pupa,  from  which  after  a  rather  definite 
interval  an  adult  emerges. 

A  combination  of  two  simple  life  cycles  to  form  one 
complex  cycle  occurs  in  certain  hydroids.  The 
eggs  of  these  species  produce  free-swimming  em- 
bryos which  become  fixed  to  some  object  and  de- 
velop into  polyps.  These  polyps  form  other  polyps 
like  themselves  by  budding,  but  finally  give  rise  to 
buds  which  become  jelly-fishes  or  medusae.  In- 
stead of  remaining  attached  to  the  parent  colony 
the  medusae,  as  a  rule,  separate  from  it  and  swim 
about  in  the  water  ;  they  later  give  rise  to  eggs  which, 
after  being  fertilized,  develop  as  before  into  polyps. 
There  are  thus  in  this  species  two  life  cycles  com- 
bined, that  extending  from  the  egg  to  the  time  when 
the  colony  forms  medusa-buds,  and  that  beginning 
with  the  medusa-bud  and  ending  with  the  mature 
egg.  Such  an  alternation  of  an  asexual  and  a  sexual 
generation  is  known  as  metagenesis. 


24  GERM-CELL   CYCLE    IN   ANIMALS 

There  is  another  sort  of  alternation  which  nor- 
mally occurs  in  many  species,  and  that  is  the  alterna- 
tion of  individuals  developing  from  parthenogenetic 
eggs  with  those  from  fertilized  eggs.  In  the  aphids, 
or  plant  lice,  for  example,  the  race  in  the  northern 
part  of  the  United  States  passes  the  winter  in  the 
shape  of  fertilized  eggs.  All  of  the  individuals  which 
hatch  from  these  eggs  in  the  spring  are  females  called 
stem-mothers.  The  stem-mothers  produce  broods  of 
females  from  parthenogenetic  eggs,  and  these  in 
turn  give  rise  to  other  broods  of  females  in  the  same 
manner.  Thus  throughout  the  summer,  generation 
after  generation  of  parthenogenetic  females  appear ; 
but  as  autumn  approaches  females  develop  whose 
eggs  must  be  fertilized,  and  males  are  also  pro- 
duced. The  eggs  of  these  females  are  fertilized  by 
spermatozoa  from  the  males,  and  the  zygotes  thus 
formed  survive  the  winter,  producing  stem-mothers 
the  following  spring. 


CHAPTER  II 

GENERAL  ACCOUNT    OF  THE    GERM-CELL    CYCLE 

IN  ANIMALS 

It  will  be  impossible  to  present  in  this  chapter  even 
a  general  account  of  all  the  variations  in  the  germ- 
cell  cycle  that  are  known  to  occur  in  animals.  It 
will  be  necessary,  therefore,  to  restrict  ourselves  to 
the  series  of  events  that  occurs  in  the  majority  of 
animals,  mentioning  as  many  of  the  more  notable 
variations  and  exceptions  as  possible  without  causing 
confusion.  It  also  seems  advisable  to  consider 
the  germ-cell  cycles  in  the  Protozoa  and  the  Meta- 
ZOA  separately. 

Protozoa.  Weismann,  in  his  classical  essays 
on  the  germ-plasm,  argues  in  favor  of  the  view  that 
the  Protozoa  are  potential  germ  cells,  and,  since  new 
individuals  arise  by  division  of  the  parent  cell  into 
two  or  more  parts,  that  natural  death  does  not  occur. 
The  Protozoa  are  consequently  also  potentially 
immortal.  The  Metazoa,  on  the  other  hand, 
possess  a  large  amount  of  somatic  substance  which 
always  dies  a  natural  death.  It  has  often  been 
pointed  out  that  a  Protozoon,  although  consisting 
of  but  a  single  cell,  performs  most  of  the  physiological 
activities  characteristic  of  the  larger,  complex 
Metazoa,  and  that  certain  parts  of  the  Protozoon 

25 

D.  H.  HILL  LIBRARY 

NJr»rt!»  ri^rolina  State  College 


26 


GERM-CELL   CYCLE   IN  ANIMALS 


Ch 


Tt 


N 


are  recognizably  concerned  with  the  performance 
of  certain  definite  functions.  The  fundamental 
difference,  then,  between  the  one-celled  and  the 
many-celled  animals  is  that  the  differentiated  struc- 
tures in  the  former  are  not  separated  from  one 
another  by  cell  walls  as  in  multicellular  organisms. 

Whether  all  Protozoa  possess  a  body  which  can  be 
considered  as  specialized  and  set  aside  for  reproduc- 
tion purposes, 
as  the  germ- 
plasm  theory 
requires,  is  a 
p  question  upon 
which  author- 
ities differ.  In 
certain  cases 
it  seems  pos- 
sible to  distin- 
guish between 
germinal  and 
somatic  proto- 
plasm without  any  difficulty.  The  life  history  of  the 
fresh  water  rhizopod,  Arcella  vulgaris  (Fig.  5),  will 
serve  to  illustrate  this  (Hertwig,  1899 ;  Elpatiewsky, 
1907;  Swarczewsky,  1908;  Calkins,  1911).  The 
single  nucleus  of  the  young  Arcella  divides  to  form 
two  primary  nuclei  (N) ;  chromatin  from  these  mi- 
grates out  and  forms  a  layer  near  the  periphery  (Ch) 
— the  "  chromidial  net "  of  Hertwig.  This  chromatin 
substance  in  the  mature  individual  produces  hundreds 
of  secondary  nuclei  (n),  each  of  which  is  cut  off,  with 


B 


Fig.  5.— Reproduction  in  Arcella  vulgaris.  A.  For- 
mation of  secondary  nuclei.  Ch  =  ehroniidia; 
n  =  secondary  nuclei;  iV  =  primary  nucleus. 
{From  Hertwig,  1899.)  B.  Two  gametes.  {From 
Elpatiewsky,  1907.) 


ACCOUNT   OF   THE   GERM-CELL   CYCLE     27 

a  small  amount  of  the  surrounding  cylopiasni, 
from  the  others,  thus  becoming  a  swarm  spore. 
The  swarm  spores  escape  from  the  mouth  of  the 
parent  cell ;  whereas  the  two  primary  nuclei  and  a 
portion  of  the  cytoplasm  not  used  up  in  the  forma- 
tion of  the  swarmers  die.  The  swarmers  are  not 
all  alike,  being  of  two  sizes ;  the  larger,  which  may  be 
called  macrogametes,  and  which  correspond  to  the 
eggs  of  the  Metazoa,  fuse  with  the  smaller  micro- 
gametes.  The  zygotes  which  result  develop  into 
normal  Arcelloe.  The  swarmers  may  be  supposed 
to  represent  the  germinal  protoplasm,  of  which,  as  in 
metazoan  germ  cells,  the  chromatin  content  may  be 
considered  the  essential  portion.  The  conditions 
during  reproduction  in  other  Protozoa  may  also  be 
explained  in  this  way,  so  that  germinal  and  somatic 
protoplasm  can  be  distinguished  as  in  the  Metazoa. 
The  discovery  of  the  chromidia  in  Protozoa 
led  to  the  formulation  of  the  hypothesis  of  binu- 
clearity.  Believers  in  this  hypothesis  maintain 
that  each  cell  contains  both  a  somatic  and  propaga- 
tory  nuclear  material  which,  as  a  rule,  are  united 
into  one  amphinucleus.  The  somatic  nuclear  ma- 
terial controls  vegetative  functions;  the  propaga- 
tive  portion  serves  only  for  the  propagation  of  new 
individuals.  Separation  occurs  rarely  except  in 
certain  Protozoa,  where,  as  in  Paramecium,  the 
propagative  substance  is  represented  by  the  micronu- 
cleus,  the  somatic  by  the  macronucleus.  Since  the 
chromatin  is  the  essential  substance  concerned  in 
the    binuclearity    hypothesis,    the    term    dichroma- 


28  GERM-CELL   CYCLE   IN   ANIMALS 

ticity  has  been  suggested  as  more  appropriate,  and 
the  two  kinds  of  chromatin  involved  have  been  called 
idiochromatin,  which  is  reproductive  in  function, 
and  trophochromatin,  which  is  vegetative  in  function. 
The  hypothesis  has  not  gained  many  adherents  and 
is  considered  of  doubtful  value  by  eminent  proto- 
zoologists  (Dobell,  1908). 

Metazoa.  If  we  consider  the  mature  egg,  either 
fertilized  or  parthenogenetic,  as  the  starting  point 
of  the  germ-cell  cycle  in  the  Metazoa,  we  may 
recognize  seven  or  eight  distinct  periods  as  follows  : 

1.  The  segregation  of  the  primordial  germ  cells; 
i.e.,  the  formation  of  one  or  more  primordial  germ 
cells  during  the  segmentation  of  the  egg ; 

2.  Early  multiplication  of  the  primordial  germ 
cells ; 

3.  A  long  period  of  "rest"  characterized  by  cessa- 
tion of  cell  division,  either  active  or  passive  change 
of  position,  separation  of  the  germ  cells  into  two 
groups  which  become  the  definitive  germ  glands, 
accompanied  by  the  general  growth  of  the  embryo 
until  the  larval  stage  is  almost  attained ; 

4.  Multiplication  by  mitosis  of  the  primitive 
oogonia  or  spermatogonia  to  form  a  definite  number 
(Miastor  and  perhaps  others)  or  indefinite  number 
(so  far  as  we  know)  of  oogonia  or  spermatogonia ; 

5.  In  some  cases  the  differentiation  of  oogonia 
into  nurse  cells  and  ultimate  oogonia,  and  the 
spermatogonia  into  Sertoli  cells  and  ultimate  sper- 
matogonia ; 

6.  The  growth  of  the  ultimate  oogonia  and  sper- 


ACCOUNT  OF   THE   GERM-CELL   CYCLE     !29 

matogonia  to   form   primary  oocytes    and   j)riniary 
spermatocytes ; 

7.  Maturation ; 

8.  Fertilization  (if  not  parthenogenetic). 

1.  The  Segregation  of  the  Primordial  Germ 
Cells.  This  phase  of  the  germ-cell  cycle  is  espe- 
cially emphasized  in  this  book  (see  Chapters  III  to  VI) 
and  need  be  referred  to  only  casually  here.  The 
mature  eggs  of  animals  are  organized  both  mor- 
phologically and  physiologically ;  that  is,  differenti- 
ations have  alread}^  taken  place  in  their  protoplasmic 
contents  before  they  are  ready  to  begin  develop- 
ment. This  organization  determines  what  sort  of 
divisions  the  *^%g  will  undergo  during  the  cleavage 
stages.  During  cleavage  certain  parts  of  the  cell 
contents  become  separated  from  other  parts  and 
thus  the  differentiated  substances  of  the  ^^g  are 
localized  in  definite  parts  of  the  embryo.  The 
contents  of  the  cleavage  cells  likewise  become 
differentiated  as  development  proceeds,  until  finally 
the  cells  produced  form  two  or  three  more  or  less 
definite  germ  layers.  In  some  cases  the  ^gg  always 
divides  in  the  same  way,  and  the  history  or  *'cell 
lineage"  of  the  cells  can  be  followed  accurately, 
and  the  parts  of  the  larva  to  which  they  give  rise 
can  be  established.  This  is  known  as  determinate 
cleavage  in  contrast  to  the  indeterminate  type  in 
which  there  appears  to  be  no  relation  between  the 
cleavage  cells  and  the  structure  of  the  ^gg  or  larva. 

The  degree  of  organization  of  the  i^gg  no  doubt  ac- 
counts for  the  differences  in  cleavage;    those  of  the 


30  GERM-CELL   CYCLE   IN   ANIMALS 

determinate  type  being  more  fully  organized  than 
those  of  the  indeterminate  type. 

The  period  v:hen  the  primordial  germ  cells  are  es- 
tablished is  probably  due  in  part  to  the  state  of 
organization  of  the  egg  when  development  begins, 
and  it  is  not  strange,  therefore,  that  the  primordial 
germ  cell  may  be  completely  segregated  in  certain 
eggs  as  early  as  the  four-cell  stage ;  whereas  in 
others  germ  cells  have  not  been  discovered  until  a 
late  larval  condition  has  been  reached.  An  ever 
increasing  number  of  species  of  animals  is  being 
added  to  those  in  wdiich  an  early  segregation  of  the 
germ  cells  has  already  been  recorded.  Neverthe- 
less, there  are  certain  zoologists  who  still  question 
the  general  occurrence  of  an  early  segregation  of 
the  germ  cells,  but  more  careful  investigations  will 
probably  establish  the  fact  of  early  segregation  in 
species  in  which  this  has  not  yet  been  demonstrated. 

2.  Early  Multiplication  of  the  Primordial 
Germ  Cells.  The  number  of  germ  cells  present 
at  the  time  of  their  first  appearance  in  the  embryo 
varies  in  different  species.  There  may  be  one,  as 
in  the  majority  of  cases,  for  example  the  fly, 
Miastor  (Fig.  17),  the  nematode,  Ascaris  (Fig.  51), 
the  crustacean,  Cyclops  (Fig.  48),  and  the  arrow 
worm,  Sagitta  (Fig.  54) ;  or  a  number,  as  in  chrysome- 
lid  beetles  (Fig.  36),  certain  parasitic  Hymenoptera 
(Fig.  44),  and  vertebrates  (Fig.  6).  As  a  rule  the 
primordial  germ  cell  or  cells  increase  in  number  by 
mitosis  soon  after  they  are  segregated,  and  then 
cease   to   divide   for   a   considerable   interval.     For 


ACCOUNT   OF  THE   GERM- CELL   CYCLE     31 

example,  in  Miastor  the  single  primordial  ^^erm  cell 
produces  eight ;  in  the  beetle  CaUu/rapha  mulfi- 
punctata  the  original  sixteen  undergo  two  divisions 
resulting  in  sixty -four ;  and  in  the  chick  Swift  (1914) 
has  counted  as  many  as  eighty-two  at  this  stage. 

We  shall  see  later  that  the  primordial  germ  cells 
are  often  characterized  by  the  presence  of  certain 
cytoplasmic  inclusions  (the  keimbahn-determinants) 
which  are  absent  from  the  other  cells  of  the  embryo. 
These  inclusions  appear  to  be  equally  divided  be- 
tween the  daughter  cells  so  that  each  of  the  eight  or 
sixty-four,  as  the  case  may  be,  is  provided  with  an 
equal  amount  of  the  keimbahn-determinants. 

3.  Period  of  "Rest"  and  Migration.  By  rest 
here  is  really  meant  cessation  of  division.  During 
this  period  the  germ  cells  either  actively  migrate 
or  are  passively  carried  by  surrounding  tissues  to 
the  position  the  germ  glands  occupy  in  the  larva. 
In  species  possessing  two  germ  glands  the  germ  cells 
separate  to  form  two  groups,  with,  at  least  in  some 
cases,  an  equal  number  in  each  group.  Thus  in 
Miastor  the  number  in  each  group  is  four  (Fig.  'I'i) 
and  in  Calligrapha,  thirty-two  (Fig.  37).  There  is 
evidence  that  an  active  migration  of  germ  cells 
occurs  both  in  vertebrates  and  invertebrates.  Figure 
6  shows  the  positions  of  the  germ  cells  in  four  s])ecies 
of  vertebrates  during  their  change  of  ])ositi()n. 
That  the  germ  cells  at  this  time  are  actively  nngrat- 
ing  by  ameboid  movements  is  the  general  opinion 
of  investigators,  since  frequently  these  cells  are 
ameboid  in  shape  and  the  distance  between  the  i)lace 


32 


GERM-CELL   CYCLE   IN  ANIMALS 


of  origin  and  the  germinal  ridge  is  too  great  to  be 
traversed  in  any  other  way. 

Professor  B.  M.  Allen,  who  has  made  extensive 
studies  of  the  germ  cells  of  many  species  of  verte- 


LepMosteus 


LepldosUus 


Fig.  6.  —  Diagrams  showing  the  paths  of  migration  in  A,  a  turtle, 
Chrysemys  marginata ;  B,  a  frog,  Rana  pipiens ;  C,  a  fish,  Lepidos- 
teus  osseus,  and  D,  the  dog-fish,  Amia  calva.  (From  Allen,  1911.) 
Arc/i  =  archenteron;  /n^  =  intestine;  La^  Mes  =  lateral  plate  of 
mesoderm;  Mes  =  mesentery;  Meson  =  mesonephros;  M2/0  =  myo- 
tome; A^oto  =  notochord;  P.  card  =  post  cardinal  vein;  S.  C  =  sex- 
cells;  S.Gl  =  sex  gland;  V it.  End  =  vitelline  endoderm;  W.D  = 
Wolffian  duct. 

brates,  makes  the  following  statement  regarding  this 
phase  of  the  germ-cell  cycle : 

"The  sex-cells  are  migratory  to  a  high  degree. 
The  path  and  time  of  their  migration  may  vary 
greatly  within  a  given  group  of  animals,  as  illus- 


ACCOUNT  OF  THE   GERM-CELL  CYCLE    33 

trated  by  the  case  of  Amia  and  Lepidosteus.  While 
in  the  forms  that  I  have  studied  they  are  first  to  be 
observed  in  the  entoderm,  I  am  quite  open  to  convic- 
tion that  in  other  forms  they  may  migrate  from  tliis 
layer  into  the  potential  mesoderm  before  the  two 
layers  are  separated,  as  shown  by  Wheeler  in  Petro- 
myzon.'^ 

Swift  (1914)  has  recently  obtained  evidence  which 
seems  to  prove  that  not  only  do  the  germ  cells  of 
the  chick  migrate  by  ameboid  movements  but  they 
enter  the  blood  vessels  and  are  distributed  by  the 
blood  stream  to  all  parts  of  the  embryo  and  vascular 


area. 


The  migration  of  the  germ  cells  has  been  noted  in 
many  invertebrates  and  has  been  fully  described 
in  chrysomelid  beetles  (Hegner,  1909a).  In  these 
insects  the  primordial  germ  cells  are  segregated  at 
the  posterior  end  of  the  egg  at  the  time  the  blasto- 
derm is  formed  (Fig.  36,  C).  The  blastoderm  is 
never  completed  just  beneath  them,  but  a  canal, 
called  the  pole-cell  canal,  remains.  Through  this  at 
a  later  embryonic  stage  the  germ  cells  migrate  by 
means  of  ameboid  movements. 

"As  soon  as  the  germ  cells  of  CalUgrapha  have 
passed  through  the  pole-cell  canal,  they  lose  their 
pronounced  pseudopodia-like  processes  and  bocuine 
nearly  spherical  (Fig.  37,  E)  ;  nevertheless,  they 
undergo  a  decided  change  in  position.  They  move 
away  from  the  inner  end  of  the  pole-cell  canal,  and 
creep  along  between  the  yolk  and  the  germ-band. 
Thus  two  groups  are  formed  near  the  developing 

D 


34  GERM-CELL   CYCLE   IN   ANIMALS 

coelomic  sacs ;  each  group  probably  contains  an 
equal  number  of  cells.  The  smallest  number  I 
have  counted  in  one  group  at  this  time  is  thirty; 
the  largest  number,  thirty-four.  As  there  is  some 
difficulty  in  obtaining  an  accurate  count,  it  seems 
probable  that  the  sixty-four  germ  cells  are  equally 
divided  and  that  each  germ  gland  receives  thirty-two. 
Some  of  the  germ  cells  migrate  not  only  laterally 
along  the  germ  gland  but  also  back  toward  the 
posterior  end  of  the  egg,  where  we  find  them  forming 
narrow  strands  in  the  last  abdominal  segments. 
From  this  stage  on,  the  germ  cells  are  not  very  active ; 
they  move  closer  to  one  another  to  form  the  compact 
germ  glands.  I  was  unable  to  determine  whether 
the  later  movements  of  the  germ  cells  are  due  to 
an  active  migration  or  to  the  tensions  created  by 
the  growth  of  the  surrounding  tissues;  the  latter 
seems  the  more  probable"  (Hegner,  1909a,  p.  280). 

It  is  thus  evident  that  during  the  blastoderm  stage 
the  germ  cells  of  this  beetle  are  actually  outside  of  the 
egg.  How  well  this  illustrates  the  theory  of  primary 
cellular  differentiation,  that  is,  the  differentiation  of 
germ  cells  from  somatic  cells,  since  the  two  sorts 
are  here  completely  separated,  the  former  constitut- 
mg  a  group  in  contact  with  but  not  connected  with 
the  somatic  cells.  Later,  as  the  germinal  con- 
tinuity hypothesis  demands,  the  germ  cells  migrate 
into  the  embryo,  there  to  be  nourished,  transported, 
and  protected  by  the  body  until  they  are  ready  to 
separate  from  the  somatic  cells,  and  thus  to  give  rise 
to  a  new  generation. 


ACCOUNT   OF   THE   GERM-CELL   CYC  LE     35 

4.  Period  of  Multiplication.  Soon  after  tlio 
germ  cells  aggregate  to  form  more  or  less  rounded 
groups  lying  in  the  position  of  the  definitive  germ 
glands  mitotic  division  is  resumed.  At  about  this 
time  also,  the  sex  of  the  individual  can  often  be 
determined  by  the  shape  of  the  germ-gland.  Then 
both  the  testes  and  the  ovaries  acquire  envelopes 
of  the  follicular  cells,  and  frequently  testicular  cysts 
and  ovarian  tubes  or  chambers  develop.  The  ques- 
tion of  the  origin  of  the  follicular  cells  is  still  un- 
settled, but  the  evidence  in  most  cases  seems  to 
favor  the  view  that  they  are  mesodermal. 

The  multiplication  of  the  germ  cells  by  mitosis 
continues  rapidly  from  this  time  on.  In  only  one 
case,  so  far  as  I  am  aware,  do  we  know  the  actual 
number  of  germ  cells  produced  by  the  primordial 
germ  cell ;  this  is  in  Miastor,  where  typically  sixty- 
four  oogonia  are  formed  (Fig.  26).  As  the  germ 
cells  multiply  they  become  smaller  in  size  and  the 
substances  present  in  the  primordial  germ  cell 
become  divided  among  a  large  number  of  progeny. 
Thus  at  the  beginning  of  the  growth  period  each 
germ  gland  contains  many  oogonia  or  spermatogonia, 
and  each  of  these  contains  a  small  fraction  of  the 
material  present  in  the  primordial  germ  cell,  plus 
whatever  substances  may  have  been  assimilated 
during  the  period  of  multiplication. 

5.  The  Origin  of  Nurse  Cells  and  Sertoli 
Cells.  Germ  cells  receive  nourishment  during  the 
growth  period  in  many  ways,  e.g.,  from  nurse  cells, 
follicle  cells,  or  directly  from  the  blood.     The  origin 


36  GERM-CELL   CYCLE   IN  ANIMALS 

of  the  nurse  cells  and  follicle  cells  is  important  since 
in  a  few  cases  the  germ  cells  themselves  are  known 
to  give  rise  to  them.  There  is  thus  a  second  differ- 
entiation whereby  somatic  cells  (follicle  cells  or 
nurse  cells)  become  differentiated  from  germ  cells 
(oogonia  or  spermatogonia).  In  some  species,  such 
as  Miastor,  we  can  prove  without  question  that  both 
the  nurse  cells  and  follicle  cells  are  of  mesodermal 
origin,  and  that  the  germ  cells  give  rise  only  to  germ 
cells.  On  the  other  hand,  there  are  instances  in 
both  vertebrates  and  invertebrates  of  a  common 
origin  of  germ  cells  and  somatic  cells  from  oogonia 
and  spermatogonia.  Perhaps  the  most  striking 
examples  are  the  differentiation  of  the  nurse  cells 
and  ultimate  oogonia  in  the  water  beetle,  Dytiscus, 
and  the  differentiation  of  the  Sertoli  cells  and  ulti- 
mate spermatogonia  in  man.  (See  Chapter  V.) 
Haecker  (1912)  distinguishes' between  a  somato-ger- 
minative  period  and  a  true  germinative  period  ;  the 
former  is  that  during  which  the  primordial  germ  cells 
are  established  and  the  latter  that  of  the  differentia- 
tion of  nurse  cells  and  ova. 

6.  The  Growth  Period.  The  last  divisions 
of  the  oogonia  and  spermatogonia  are  followed  by 
the  growth  of  these  cells.  The  extent  of  this  growth 
depends,  in  the  case  of  the  female,  upon  whether 
or  not  the  mature  egg  is  to  be  supplied  with  an 
abundance  of  nutritive  material.  Nurse  cells,  fol- 
licle cells,  and  circulating  fluids  may  all  assist  in  the 
enlargement  of  the  oogonia.  If  the  eggs  are  small, 
sufficient    nutriment    is    supplied    by    surrounding 


ACCOUNT   OF   THE   GERM-CELL   C  YCLE     37 

liquids  and  no  special  nurse  cells  are  required  ;  but 
larger  eggs  either  become  surrounded  by  follicle  cells 
which  nourish  them  and  with  which  they  are  often 
intimately  connected  by  protoplasmic  bridges,  or 
special  nurse  cells  are  provided.  In  the  primitive 
type  of  ovary,  such  as  exists  in  most  coelenterates, 
any  of  the  cells  surrounding  the  oogonium  may 
function  as  nurse  cells  and  even  neighboring  oogonia 
are  engulfed  by  the  oogonium  that  is  successful  in  the 
struggle  for  development.  A  more  definite  mechan- 
ism exists  in  higher  organisms,  where  one  or  more 
cells  become  differentiated  for  the  special  purpose  of 
supplying  nutriment  consisting  of  either  their  own 
substance  or  of  material  elaborated  by  them  and 
then  transferred  to  the  egg.  The  egg  of  the  annelid, 
Ophryotrocha,  for  example,  is  accompanied  by  a  single 
nurse  cell ;  that  of  Myzostoma  is  provided  with  two, 
one  at  either  end ;  and  the  eggs  of  certain  insects 
are  more  or  less  intimately  connected  with  groups  of 
cells  in  definite  nurse  chambers  (Fig.  46). 

The  growth  of  an  oogonium  may  be  well  illus- 
trated by  that  of  the  potato  beetle. 

The  general  arrangement  of  the  cells  in  the  ovary 
of  an  adult  beetle  is  shown  in  Fig.  7.  The  terminal 
chamber  of  the  ovarian  tubule  contains  three  kinds 
of  cells:  (1)  nurse  cells  (/i.c),  (2)  young  oocytes 
{y.o)  and  growing  oocytes,  and  (3)  epithelial  cells. 
The  nurse  cells  and  oocytes  are  both  derived  from  the 
oogonia  ;  the  epithelial  cells  are  of  mesodermal  origin. 

The  positions  of  the  stages  to  be  described  are 
indicated  in  the  diagram  (Fig.  7)  and  the  nuclear 


38 


GERM-CELL  CYCLE  IN  ANIMALS 


ooc 


Fig.  7.  —  Leptinotarsa  de- 
cemlineata.  Diagram  of 
an  ovarian  tubule  showing 
various  stages  in  the  de- 
velopment of  the  oocyte. 
The  capital  letters  refer  to 
the  positions  f  cells  shown 
in  Fig.  8.  cy  =  cytoplasm; 
es  =  egg  string;  n.c  =  nurse 
chamber;  doc  =  oocyte;  y.o 
=  young  oocyte. 


and  cytoplasmic  structures 
are  shown  in  Fig.  8.  Two 
oocytes  and  a  neighboring 
epithehal  cell  from  position 
A  in  Fig.  7  are  shown  in  Fig. 

The  nuclei  of  the  oocytes 
are  large  and  contain  a  dis- 
tinct spireme ;  the  cytoplasm 
is  small  in  amount  and  ap- 
parently homogeneous.  After 
a  short  period  of  growth  the 
oocytes  form  a  linear  series 
in  the  ovarian  tubule  and 
become  connected  with  the 
spaces  between  the  nurse  cells 
by  means  of  egg  strings  (Fig. 
7,  e.s)  through  which  the  nu- 
tritive streams  flow  into  the 
oocytes.  One  of  the  young- 
est of  these  oocytes  is  repre- 
sented in  Fig.  8,  B  (position 
B  in  Fig.  7).  The  nucleus  is 
no  larger  than  in  those  of  the 
earlier  stage ;  its  chromatin 
forms  a  reticulum,  and  a  dis- 
tinct nucleolus  is  present. 
The  cytoplasm,  on  the  other 
hand,  has  trebled  in  amount 
and  within  it  are  embedded 
a  number  of  spherical  bodies 


ACCOUNT  OF  THE   GERM-CELL  CYCLE    30 


:.  J'i 


'.■■> 


•c^ 


;.:.,W-_     -r;*         ::.•.•,:.•.^vv.■..:..:•... 


% 


/ep-|: 


-TIG.  8.       Leptinotarsa  decemlineafn       a^tt  q.  .     ,, 

oocyte  from  positions  rcHcatod  il  I^'.    7^''  "^  ''^^'  ^'"^^'^'^  "^  ^''" 
division  of  nurse  cells      ./  -  I  .^      '  ''~''  =  anutotic  nuc-l.ar 

iur^e  cells,     ch  =  chorion;  f.ep  =  follicular  epithelium. 


40  GERM-CELL   CYCLE   IN  ANIMALS 

which  stain  with  crystal  violet  after  Benda's  method, 
and  appear  to  be  mitochondrial  in  nature.  At  a 
slightly  later  stage  (Fig.  8,  C ;  position  C  in  Fig.  7) 
the  nucleus  is  larger  and  contains  several  small 
spherical  chromatic  bodies  besides  the  nucleolus. 
The  cytoplasm  has  increased  more  rapidly  in  volume 
and  a  corresponding  increase  in  the  number  of  mito- 
chondrial granules  has  also  taken  place.  Further 
growth  results  in  an  increase  in  the  volume  of  both 
nucleus  and  cytoplasm  (Fig.  8,  D ;  position  D  in 
Fig.  7),  and  a  slight  increase  in  the  number  of  mito- 
chondria. Whether  these  bodies  developed  de  novo 
or  by  division  of  the  preexisting  granules  could  not 
be  determined. 

In  succeeding  stages  growth  is  very  rapid.  The 
cytoplasm  (Fig.  8,  E\  position  E  in  Fig.  7)  still 
remains  homogeneous  except  for  the  mitochondria, 
which  increase  slightly  in  size  and  become  situated 
as  a  rule  near  the  periphery.  The  nucleus  at  this 
time  contains  a  large  number  of  chromatin  granules 
and  a  diffuse  reticulum.  Part  of  an  older  oocyte 
is  shown  in  Fig.  8,  F  (position  F  in  Fig.  7) ;  the  cyto- 
plasm has  assumed  a  reticular  appearance;  the 
mitochondrial  granules  are  present  in  greater  num- 
bers, and  the  nucleus  is  larger,  oval  in  shape,  and 
contains  a  distinct  reticulum  with  manv  chromatin 
bodies  of  various  sizes.  A  still  older  oocyte  (Fig. 
8,  G ;  position  G  in  Fig.  7)  is  interesting  particularly 
because  of  the  rapid  increase  in  the  mitochondria  and 
the  localization  of  these  near  the  periphery.  From 
this  stage  on  the  character  of  the  contents  changes 


ACCOUNT   OF   THE   GERM-CELL   CYCLE     41 

until,  as  shown  in  Fig.  7,  the  central  part  of  the 
oocyte  consists  of  homogeneous  cytoplasm  (cz/),  and 
the  outer  region  of  the  cytoplasm  is  crowded  with 
granules  and  spherical  bodies  of  various  sizes. 
Apparently  the  mitochondria  lying  near  the  periphery 
(Fig.  8,  H)  increase  in  size,  gradually  losing  their 
affinity  for  the  crystal  violet  stain  and  swelling  up 
until  they  constitute  the  large  yolk  globules  so 
numerous  in  the  mature  egg.  All  stages  in  the 
evolution  of  these  bodies  are  illustrated  at  this  time 
as  represented  in  Fig.  8,  H.  Li  the  meantime 
material  is  brought  into  the  egg  through  the  egg 
string  from  the  nurse  cells,  thus  probably  adding 
several  sorts  of  granules  to  the  contents  of  the  oocyte. 

The  growth  period  in  the  male  germ-cell  cycle  is 
not  so  striking  as  in  the  female,  since  many  sperma- 
tozoa of  small  size  are  produced,  whereas  only 
comparatively  few  large  eggs  develop.  An  increase 
in  the  size  of  the  ultimate  spermatogonia  may  occur, 
however,  but  the  multiplication  and  growth  periods 
are  not  nearly  so  distinct  as  in  the  case  of  the  oogonia. 
In  testes  which  are  composed  of  cysts  of  spermato- 
gonia there  is  evidence  in  some  cases  that  all  of  the 
germ  cells  in  a  single  cyst  are  descendants  of  a  single 
spermatogonium.  The  proof  for  this  seems  certain 
in  the  potato  beetle,  where  I  have  been  al)k>  to 
follow  the  formation  of  the  cysts  by  means  of  an 
uninterrupted  series  of  stages  (Hegner,  1914a). 

7.  Maturation.  Maturation  or  the  ripening 
of  the  eggs  and  spermatozoa  comprises  a  series  of 
events  which  results  in  a  reduction  in  the  number 


42 


GERM-CELL   CYCLE   IN  ANIMALS 


of  chromosomes  and  the  amount  of  chromatin  in 
the  germ  cells.  Typically,  both  male  and  female 
germ  cells  divide  twice  during  the  process  of  matura- 


PRIMORDIAL 
GERM-CELL 


MULTIPLICATION 
•    •  \  f  •  •   ^  r  PERIOD 

SPFRMATOGONIA^--"'  \  •  •  /  V  •  • 

•  v       <!_V       V^V       Uv 

GROWTH 
PERIOD 
PRIMARY  /    t    1 

SPERMATOCYTE  '      * 

SECONDARY  .      ,  ,     ^, 

SPERMATOCYTES 

MATURATION 
_  PERIOD 

SPERMATIDS  (7>\      Cr>\    (i 

SPERMATOZOA  ^ 

PRIMORDIAL  , 

GERM-CELL  \%  * , 

MULTIPLICATION 
•  •>  f  *  •  '^  ^  PERIOD 

OOGONIA  ^' 

N 

N 

N 

s 

'••\  '  ^  *\  [*  *\  { *  * 

f  J  V •  •/  \«  •/  \  •  •> 

GROWTH 
Pf'l^'ARY  /  \  ^     pERioo 

OOCYTE  I       •    • 

SECONDARY 

OOCYTES 

(OVARIAN  EGG    ""  ~      ~  ~\     *    *     }  ^  V     MATURATION 

AND  POLARBODY)  \  7  /\  >  PERIOD 

MATURE  EGG  /^       ^    \^  d.  \_^ 

AND  /^     ,    ,     ]    P)      rt  ft 

POLAR  BODIES  \  7    ^^     V_y  ^-^ 

Fig.  9.  —  Diagrams  illustrating  (above)  the  stages  of  spermatogenesis 
and  (below)  of  oogenesis.  The  primordial  germ  cell  is  represented 
as  possessing  four  chromosomes. 


ACCOUNT  OF  THE   GERM-CELL  CYCLE    43 

tion,  and  as  shown  in  Fig.  9  these  divisions  result 
in  the  production  of  four  functional  spennalozoa 
in  the  male,  and  one  functional  egg  and  three  polar 
bodies  (abortive  eggs)  in  the  female.  This  increase 
in  the  number  of  cells  is  not,  however,  the  most  im- 
portant phase  of  the  maturation  process,  since  a 
large  part  of  our  knowledge  of  the  physical  l^asis  of 
heredity  has  been  derived  from  studies  of  the  be- 
havior of  the  chromatin  at  this  time.  This  subject 
will  be  dealt  with  more  fully  in  Chapter  IX,  and 
for  the  present  only  a  brief  account  of  events  need 
be  given. 

The  first  thing  to  be  noted  is  that  the  mitoses 
leading  to  the  division  of  the  germ  cells  during  mat- 
uration differ  from  those  of  ordinary  cell  multiplica- 
tion. The  germ  cells,  when  they  are  ready  for  the 
maturation  divisions,  are  known  as  primary  oocytes 
and  primary  spermatocytes.  The  nuclei  of  these 
cells  possess  the  complete  or  diploid  number  of 
chromosomes,  characteristic  of  somatic  cells ;  but 
after  maturation  the  eggs  and  spermatozoa  con- 
tain only  one-half  of  the  original  diploid  number, 
or  the  haploid  number.  These  mitoses  are  conse- 
quently called  reducing  or  meiotic.  The  details  of 
these  mitoses  differ  in  male  and  female  germ  cells 
and  in  different  species  of  animals. 

During  and  at  the  close  of  the  growth  period  in  the 
male  the  chromatin  granules  form  a  spireme  which 
condenses  at  one  side  of  the  nucleus,  a  condition 
known  as  synizesis.  After  a  time  the  s})irenie 
again  spreads  throughout  the  nucleus,   but   is  now 


44  GERM-CELL   CYCLE   IN  ANIMALS 

divided  into  segments,  the  chromosomes,  which  are 
only  haploid  in  number.  The  reduction  from  the 
diploid  to  the  haploid  number  is  brought  about  by 
the  union  of  the  chromosomes  in  pairs,  a  condition 
called  synapsis.  Each  of  the  haploid  chromosomes 
thus  consists  of  two  of  the  diploid  chromosomes 
and  is  said  to  be  bivalent.  That  one  of  the  chromo- 
somes of  each  pair  is  of  maternal  origin,  i.e.,  is  a 
descendant  of  a  chromosome  present  in  the  egg  at 
the  time  of  fertilization,  and  the  other  of  pater- 
nal origin,  i.e.,  a  descendant  of  one  brought  into 
the  egg  by  the  spermatozoon,  seems  to  be  well 
established.  The  final  act  of  fertilization,  therefore, 
occurs  at  this  point  in  the  germ-cell  cycle  —  an 
act  of  much  greater  significance  than  that  of  the 
union  of  the  egg  and  spermatozoon.  Furthermore, 
there  is  considerable  evidence  that  the  chromo- 
somes differ  one  from  another  and  that  in  synapsis 
corresponding  (homologous)  chromosomes  unite. 
The  importance  of  such  a  union  from  a  theoretical 
standpoint  will  be  discussed  later. 

The  nuclei  now  prepare  for  the  two  maturation 
mitoses.  In  manv  nematodes,  annelids,  and  arthro- 
pods  these  are  characterized  by  the  formation  of 
tetrads.  Divisions  of  this  sort  may  be  illustrated 
as  in  Fig.  10.  The  diploid  number  of  chromosomes 
is  for  convenience  supposed  to  be  four,  as  in  the  sper- 
matogonium A.  During  the  spermatogonial  divi- 
sions these  divide  as  in  B,  so  that  each  daughter  cell 
receives  the  diploid  number,  four.  After  synapsis, 
however,  each  of  the  haploid  chromosomes  of  the 


ACCOUNT   OF   THE   GEKM-CELL   CY(  LE     45 


Fig.  10.  —  Diagrams  showing  the  (.^sential  facts  of  ri'tluction  in  the 
male.  The  somatic  number  of  chromosomes  is  supposeil  to  bo  four. 
A,  B,  division  of  the  spermatogonia,  showing  the  full  number  (four) 
of  chromosomes.  C,  primary  spermatocyte  i^rcparing  for  division: 
the  chromatin  forms  two  tetrads.  D,  E,  F,  first  division  to  form 
.  two  secondary  spermatocytes,  each  of  which  receives  two  dyads. 
G,  H,  division  of  the  two  secondary  spermatocytes  to  form  four 
spermatids.  Each  of  the  latter  receives  two  .><ingl('  chromnsonies  and 
a  centrosome  which  passes  into  the  middle  piece  of  the  spermatozoon. 
(After  Wilson.) 


46  GERM-CELL   CYCLE   IN   ANIMALS 

primary  spermatocyte  is  seen  to  be  divided  into 
four  parts,,  thus  forming  in  this  case  two  tetrads  (C). 
During  the  division  of  the  primary  spermatocyte, 
as  shown  in  /),  E,  and  F,  half  of  each  tetrad,  or  two 
dyads,  passes  to  each  daughter  cell.  The  division 
of  the  daughter  cells,  which  are  known  as  secondary 
spermatocytes  {G  H),  results  in  the  separation  of  the 
two  parts  of  each  dyad  so  that  each  of  the  four 
spermatids  {H)  receives  one  member  of  each  original 
tetrad  or  two  monads.  Thus  the  chromosomes 
(monads)  of  the  spermatids  {H)  are  already  formed 
in  the  primary  spermatocytes  (C)  by  two  divisions ; 
whereas  the  nuclear  and  cell  divisions  do  not  occur 
until  later.  The  spermatids  {H),  which  proceed 
to  metamorphose  into  spermatozoa,  possess,  there- 
fore, only  two  chromosomes,  i.e.,  one-half  of  the 
number  present  in  the  spermatogonia  {A)  and  so- 
matic cells. 

Tetrad  formation  does  not  occur  in  most  animals ; 
but  usually  the  members  of  the  bivalent  chromosomes 
become  separated  on  the  first  maturation  spindle, 
the  pairs  appearing  U-,  V-,  or  ring-shaped,  as  in 
Fig.  6^.  Each  secondary  spermatocyte  receives 
one-half  of  each  haploid,  bivalent  chromosome.  The 
second  maturation  mitosis  then  ensues,  during  which 
each  daughter  cell  is  provided  with  one-half  of  each 
chromosome  as  in  ordinary  mitotic  division.  Be- 
cause of  the  peculiar  behavior  of  the  chromosomes 
the  first  division  is  often  called  the  heterotype, 
whereas  the  second  is  known  as  the  homotype  divi- 
sion.    The  final  results  are  the  same  whether  tetrads 


ACCOUNT   OF   THE   GERM-CELL   CYCLE     47 

are  formed  or  not,  each  spermatid  containing  the 
haploid  number  of  chromosomes. 

The  maturation  of  the  egg  differs  in  no  very  im- 
portant respects  from  the  process  as  it  has  been 
described  in  the  male  cells.  Tetrads  may  or  may 
not  be  formed  according  to  the  species,  and  the 
mature  egg  and  polar  bodies  each  contain  the  haploid 
number  of  chromosomes.  Two  phases  of  the  matura- 
tion of  the  egg  may  be  referred  to  here  :  (1)  when 
the  nucleus  of  the  primary  oocyte  prepares  for  divi- 
sion a  considerable  amount  of  chromatin  separates 
from  the  chromosomes  and  is  lost  in  the  cytoplasm. 
The  size  of  the  chromosomes  is  thus  diminished,  but 
no  entire  chromosomes  are  lost.  (2)  The  cellular 
divisions  are  very  unequal,  the  polar  bodies  being 
very  small  as  compared  with  the  rest  of  the  egg. 
The  chromatin  content  of  the  polar  bodies,  however, 
is  equal  to  that  of  the  much  larger  egg.  In  the  male 
all  of  the  four  spermatids  are  functional,  but  in  the 
female  only  the  egg  survives,  the  polar  bodies  i\e- 
generating.  As  a  rule  two  polar  bodies  are  produced, 
but  in  certain  cases  of  parthenogenesis  (rotifers, 
Cladocera,  Ostracoda,  and  aphids)  only  one  is 
formed.     Rarely  the  first  polar  body  divides  into  two. 

8.  Fertilization.  Eggs  that  develop  ])arthen()- 
genetically  are  ready  to  begin  a  new  germ-cell  cycle 
as  soon  as  they  become  mature;  but  the  eggs  of 
the  majority  of  species  must  be  fertilized  before^ 
they  are  able  to  develop.  Fertilization  mny  be  de- 
fined as  the  fusion  of  an  egg  with  a  spermatozoiin  and 
the  resulting  processes  of  rearrangement  of  the  egg 


48  GERM-CELL   CYCLE   IN  ANIMALS 

contents  which  result  in  the  formation  of  a  uninuclear 
cell,  the  zygote.  As  a  rule  one  spermatozoon  only 
enters  the  egg  (monospermy) ;  but  in  a  few  species 
(certain  insects,  selachians,  tailed  amphibians,  reptiles, 
and  birds)  many  spermatozoa  may  normally  fuse 
with  the  egg  (physiological  polyspermy).  The  sper- 
matozoon, which  consists  usually  of  three  rather  dis- 
tinct parts,  the  head,  the  middle  piece,  and  tail, 
may  become  entirely  embedded  within  the  egg  sub- 
stance, or  the  tail  may  be  left  outside,  or,  in  excep- 
tional cases,  only  the  head  succeeds  in  entering. 

The  union  of  the  egg  and  spermatozoon  may  occur 
before,   during,  or  after  the  polar  body  formation 
(Fig.   11).     If  the  spermatozoon  enters  before  the 
maturation  of  the  egg  is  completed   (A),  its  head 
transforms  into  a  nucleus  equal  in  size  to  that  of  the 
egg  (C)  ;    the  middle  piece  dissolves,  giving  rise  to 
a  centrosome  which  inaugurates  the  formation  of  a 
spindle   with   asters    (B)  ;      and    the    tailpiece   ap- 
parently takes   no   active  part   in   the  fertilization 
processes.     The  middle  piece  also  does  not  seem  to 
be  necessary  for  the  formation  of  the  centrosomes 
and  asters.     The  nucleus  of  the  spermatozoon  and 
that  of  the  mature  egg  approach  each  other  and 
come  into  contact  between  the  asters  (C).     Then  the 
nuclear  walls  dissolve;    a  spireme  which  segments 
into  the  haploid  number  of  chromosomes  is  produced 
by  each  nucleus,  and  the  first  cleavage  spindle  of 
the  developing  egg  results.     This  spindle  bears  the 
haploid  number  of  chromosomes  from  the  spermato- 
zoon   and    a    like   number   from    the    egg    nucleus 


ACCOUNT   OF   THE   GERM-CELL   CYCLE     41) 

and  thus  the  diploid  or  somatic  number  of  diromo- 
somes  is  regained. 

When  the  spermatozoon  enters  an  egg  which  has 
completed  polar-body  formation,  the  head  does  not 


Fig.  11. —  Diagrams  of  two  principal  tj-pcs  of  fertilization.  I.  Polar 
bodies  formed  after  the  entrance  of  the  spermatozoa  (annelids, 
mollusks,  flat-worms).  II.  Polar  bodies  formed  before  entrance 
(echinoderms). 

A,  sperm-nucleus  and  centrosome  at  $  ;  first  polar  body  forming 
at  9  •  B,  polar  bodies  formed;  approach  of  the  nuclei.  C,  union 
of  the  nuclei.  D,  approach  of  the  nuclei.  E,  union  of  the  nuclei. 
F,  cleavage-nucleus.     {After  Wilson.) 

have  time  to  transform  into  a  nucleus  as  large  as 
the  egg  nucleus,  but  nevertheless  fuses  with  the  latter 
(Fig.  11,  D,  E,  F).  Although  the  two  nuclei  are  very 
unequal  in  size,  they  possess  an  equal  amount  of 
chromatin  and  furnish  an  equal  number  of  chromo- 
somes to  the  first  cleavage  spindle. 

E 


50  GERM-CELL  CYCLE  IN  ANIMALS 

As  already  indicated,  perhaps  the  most  essential 
phase  in  the  fertilization  process  does  not  occur  until 
the  homologous  maternal  and  paternal  chromosomes 
unite  during  synapsis,  when  the  germ  cells  of  the 
new  individual  become  mature.  The  immediate 
results  of  fertilization  are :  (1)  the  inauguration  of 
the  development  of  the  egg,  (2)  the  increase  of  the 
chromosomes  from  the  haploid  to  the  diploid  (so- 
matic) number,  and  (3)  the  union  of  hereditary 
substances  from,  as  a  rule,  two  individuals. 

This  completes  the  last  stage  in  the  germ-cell 
cycle  of  animals.  Many  extremely  important  and 
interesting  phases  of  the  subject  have  had  to  be 
omitted  from  the  account.  Certain  of  these  will 
be  more  fully  discussed  in  succeeding  chapters,  es- 
pecially those  concerned  with  the  early  history  of 
the  germ  cells  during  embryological  development, 
but  for  the  details  of  the  nutrition,  growth,  matura- 
tion, and  fertilization  of  the  germ  cells,  the  reader 
must  be  referred  to  other  sources  (Wilson,  1900; 
Jenkinson,  1913;    Kellicott,  1913). 


CHAPTER  III 

THE    GERM-CELL    CYCLE    IN    THE    PiEDOGENETIC 

FLY,    MIASTOR 

Thus  far  in  only  one  genus  of  animals  has  the 
history  of  the  germ  cells  from  one  generation  to  the 
next  been  followed  in  detail  through  the  entire 
cycle.  This  is  a  genus  of  flies,  Miastor,  of  the  family 
Cecidomyidae.  One  species,  Miastor  metraloas,  oc- 
curs in  Europe  and  has  there  been  studied  especially 
by  Leuckart  (1865),  Metschnikoff  (1865,  1866),  and 
Kahle  (1908),  and  the  only  other  species  that  has 
been  investigated  is  M.  americana  (Hegner,  \^V2, 
1914a). 

Psedogenesis  in  Miastor  was  discovered  by  Wagner 
in  1862,  and  was  confirmed  by  Meinert  in  1864. 
In  1865  the  first  investigations  of  its  embryological 
development  were  published  by  Leuckart  and  Metsch- 
nikoff. These  were  the  earliest  accounts  of  the 
keimbahn  in  any  animals.  Only  a  glance  at  Metsch- 
nikoff's  report  is  necessary  to  convince  one  of  the 
favorableness  of  Miastor  as  material  for  germ-cell 
studies.  The  primordial  germ  cell  is  shown  to  be 
established  at  a  very  early  period  in  the  cleavage  of 
the  egg,  and  the  descendants  of  the  primordial  germ 
cell  are  quite  easily  distinguishable  from  other  cells 
in  the  body  even  in  in  toto  preparations.     In  spite  of 

51 


52  GERM-CELL  CYCLE  IN  ANIMALS 

the  work  of  the  above  named  investigators  there  were 
many  who  were  not  convinced  that  psedogenesis 
occurs  in  the  genus,  and  the  larvse  which  were 
known  to  develop  within  the  bodies  of  other  larvae 
were  considered  by  these  skeptics  as  parasites.  How- 
ever, the  results  of  Kahle's  (1908)  studies,  which  have 
been  decisively  confirmed  (Hegner,  1912,  1914a), 
have  finally  settled  the  question  in  favor  of  psedogen- 
esis. 

Previous  to  1910  no  specimens  of  the  genus 
Miastor  had  been  recognized  in  this  country,  but 
on  Oct.  5  of  that  year,  Dr.  E.  P.  Felt  found  them  in 
great  abundance,  living  in  the  partially  decayed 
inner  bark  and  in  the  sapwood  of  a  chestnut  rail. 
With  material  supplied  by  Dr.  Felt,  the  writer 
has  been  able  to  follow  the  entire  keimbahn  in  these 
insects.  P^dogenetic  reproduction  normally  oc- 
curs during  the  spring,  summer,  and  autumn,  multi- 
plication being  arrested  during  the  cold  winter 
months.  This  method  of  reproduction  is  interrupted 
in  midsummer  by  the  appearance  of  male  and  female 
adults. 

The  larva  of  Miastor  possesses  two  ovaries,  one  on 
either  side  of  the  body  in  the  tenth  or  eleventh 
segment.  Each  ovary  (Fig.  12)  consists  of  typically 
thirty-two  oocytes  (odc.n)  ;  these  are  inclosed  in  a 
cellular  envelope  (en) .  Associated  with  each  oocyte 
is  a  group  of  mesoderm  cells  which  function  as  nurse 
cells  (n.c.)  and  together  with  the  oocyte  are  sur- 
rounded by  a  follicular  epithelium  (f.ep).  The 
nurse  cells  furnish  nutrition  to  the  growing  oocytes, 


THE   P.EDOGENETIC   FLY,   MIASTOR       53 

gradually  becoming  reduced  as  the  oocytes  increase  in 
size.  Finally  the  oocyte  (and  accompanying  nurse 
cells),  still  surrounded  by  the  follicular  epithelium, 


Fig,   12.  —  Miastor  americana.     Longitudinal  section  through  an  ovary. 

en  =  envelop;     /.fp  =  follicular    epithelium;     n.c  =  nurse    chanihcr; 

n.c.n  =  nurse-cell  nucleus;  o.m  =  mesoderm;  ouc.ti  =  o(')cyte  nucleus. 
Fig.   13.  —  Miastor  americana.     Longitudinal  section  through  a  nearly 

full-grown    oocyte,      g.v  =  germinal    vesicle;    n.c  =  nurse    chamlMT; 

pPl  =  pole-plasm. 


54  GERM-CELL  CYCLE   IN  ANIMALS 

becomes  separated  from  the  rest  of  the  ovary  and  is 
forced  by  the  movements  of  the  larva  into  some  other 
part  of  its  body.  Here  it  continues  its  growth  and 
development  at  the  expense  of  the  tissues  of  the 
mother-larva.  Not  all  of  the  oocytes  (thirty-two 
in  each  ovary)  complete  their  development,  since 
usually  only  from  five  to  seventeen  young  are 
produced  by  a  single  mother-larva.  Those  oocytes 
that  do  not  perish  pass  through  the  stages  described 
in  the  following  paragraphs. 

Figure  13  represents  the  condition  of  an  oocyte  just 
before  the  initiation  of  the  maturation  processes. 
The  nucleus,  or  germinal  vesicle  (g.v.),  is  eccentrically 
placed  and  nearer  the  anterior  than  the  posterior 
end  of  the  cell.  The  nurse  chamber  has  greatly 
decreased  in  volume. 

The  contents  of  the  oocyte  are  not  homogeneous, 
but  several  distinct  regions  can  be  distinguished. 
Near  the  nurse  chamber  is  a  body  of  cytoplasm 
evidently  elaborated  by  the  nurse  cells,  and  at  the 
posterior  end  is  an  accumulation  which  we  may  call 
the  pole-plasm  (pPl)  and  which  is  of  particular 
interest  since  it  is  intimately  associated  with  the 
formation  of  the  primordial  germ  cell. 

The  maturation  division  occurs  soon  after  the 
stage  just  described  has  been  attained.  The  ger- 
minal vesicle,  which  lies  near  the  periphery  of  the 
oocyte,  breaks  down,  and  the  chromatin  contained 
within  it  becomes  aggregated  into  about  twenty 
chromosomes.  As  a  result  of  the  maturation  division 
(Fig.  14)  a  polar  body  (p.b)  and  the  female  pronucleus 


THE  P^DOGENETIC  FLY,  MIASTOR       55 

(f.n)  are  produced.     The  nucleus  of  the  pohir  body 
divides  by  mitosis  and  the  two  nuclei  thus  formed 


Fig.  14.  —  Miastor  americana.  Longitudinal  section  tlirougli  mature 
egg.  c  =  cytoplasm;  /.?i  =  female  nucleus;  n.c  =  nurse  chamber; 
p.b  —  polar  bodies;  pP^  =  pole-plasm. 

remain  within  the  egg  substance  near  the  periphery 
for    a    considerable    period    (Fig.    14),    but    finally 


CMp 


56  GERM-CELL   CYCLE   IN   ANIMALS 

disintegrate     and    disappear,     apparently     without 
performing  any  function.     As  in  most  other  animals, 
these  polar  bodies  may  be  considered  abortive  eggs. 
The  female  pronucleus  moves  into  the  central  an- 
terior   part    of 
the  e^g  where 
it  becomes  em- 
bedded  in  the 
cytoplasmic 
mass  near  the 
nurse  chamber. 
It  may  now  be 
designated     as 
the  cleavage 
nucleus,     since 
the    eggs     of 
Mia^^or  develop 
without     ferti- 
lization   and 
hence  no  male 
pronucleus    is 
ppl    present  to  unite 
with    it.     The 

jTjQ    15  — Miastor  mctraloas.     Three  of  the  four  clcavagC    dl- 

division  figures  (I,  III,  IV)  of  the  four- to  eight-  yjgJQns    take 
cell     stage     represented.       cMp  =  chromosome 

middle  plate  ;  n.c  =  nurse  chamber  :   p.b  =  polar  place      by      Hll- 

body ;  pPl  =  pole-plasm.      {From   Kahle,    1908.)  ^^^^^       ^^^       ^^ 

in  most  of  the  Arthropoda,  the  early  cleavage  nuclei 
are  not  separated  by  cell  walls,  but  simply  move 
apart  after  each  successive  division.  The  egg  during 
this  period  is  thus  a  syncytium  within  which  the 
limits  of  the  cells  are  difficult  to  define. 


THE   P^DOGENETIC   FLY,   MIASTOR       57 

The  nuclei  present  at  the  four-cell  stage  occupy 
rather  definite  positions  and  may  be  numbered  for 
convenience  by  the  Roman  numerals  I,  II,  III, 
and  IV,  as  indicated  in  Fig.  15.  The  division  from 
the  four-  to  the  eight-cell  stage  is  a  very  important 
one,    since   it   is   at   this   time   that  the  primordial 


r.»>.c;-...-.:..;:h 


A^-^-iji'/itcV*'"''-'' 


'i'l- 


V' 

Fig.  16.  —  Miastor  metraloas.     Stages  in  the  chromatin-diminution 

process.     {From  Kahle,  1908.) 

germ  cell  is  established.  Each  of  the  four  nuclei 
divides  by  mitosis,  but  nuclei  I,  II,  and  III  undergo 
a  chromatin-diminution  process  during  which  a 
large  part  of  their  chromatin  remains  in  the  cyto- 
plasm when  the  daughter  nuclei  reform.  The  details 
of  such  a  process  are  indicated  in  Fig.  16.  Nucleus 
IV,  on  the  other  hand,  divides  as  usual  (Fig.  15)  and 
each  daughter  nucleus  receives  one-half  of  its  chroma- 
tin. One  of  these  daughter  nuclei  becomes  embedded 
in  that  peculiar  mass  of  cytoplasm  at  the  posterior 


M  p 


58 


GERM-CELL   CYCLE   IN  ANIMALS 


end  which  we  have  called  the  pole-plasm,  and  ap- 
parently  all   of  the  pole-plasm,  together  with  this 


CM  p 


cR- 


Fig.  17.  —  Miastor  americana.  Longitudinal  section  of  egg  with  one 
germ  cell  (p.g.c.)  and  nuclei  undergoing  chromatin-diminution  pro- 
cess, c  =  cytoplasm  ;  c Af  p  =  chromosome  middle  plate;  cR  = 
chromatin  remains. 

nucleus,  is  then  cut  off  from  the  egg  (Fig.  17).     This 
cell,  as  has  been  conclusively  proven  by  studies  of 


THE  PiEDOGENETIC  FLY,   MIASTOR       59 

later  stages,  is  the  primordial  germ  cell.  At  this 
time,  then,  the  egg  consists  of  one  primordial  germ 
cell  provided  with  a  nucleus  with  an  undiminished 
amount  of  chromatin,  and  a  syncytium  containing 
seven  nuclei  of  which  the  sister  nucleus  of  the  primor- 
dial germ  cell  contains  a  complete  supply  of  chroma- 
tin, whereas  the  other  six  nuclei  have  lost  part  of 
this  chromatin  material.  Reference  to  the  diagram 
on  page  65  will  assist  in  making  more  clear  this  stage 
and  the  stages  yet  to  be  described. 

The  next  developmental  process  is  the  mitotic 
division  of  the  seven  nuclei  in  the  syncytium  thus 
producing  a  fifteen-cell  stage  (Fig.  17).  The  sister 
nucleus  of  that  of  the  primordial  germ  cell  now  under- 
goes a  chromatin-diminution  process  and  the  other 
six  nuclei  in  the  syncytium  pass  through  a  second 
chromatin-diminution  process.  As  a  result  every 
nucleus  in  the  egg  has  lost  a  part  of  its  chromatin 
except  that  of  the  primordial  germ  cell  which  still 
contains  a  complete  amount.  The  further  history 
of  the  somatic  nuclei  does  not  differ  essentially 
from  that  of  the  somatic  nuclei  in  other  insects. 
They  increase  in  number  by  mitosis,  migrate  to 
the  periphery,  and  there  are  cut  off  by  cell  walls 
forming  a  single  layer  of  cells  over  the  entire  surface 
except  where  interrupted  at  the  posterior  end  by 
the  primordial  germ  cells.  Next,  a  thickening  of 
the  cells  occurs  on  the  ventral  surface,  thus  forming 
the  ventral  plate.  From  this  plate  most  of  the 
embryo  arises ;  it  lengthens  until  the  anterior  or 
cephalic  end  almost  reaches  the  anterior  end  of  the 


60  GERM-CELL   CYCLE   IN   ANIMALS 

egg,  and  until  the  posterior  or  tail  end  has  been 
pushed  around  for  a  considerable  distance  on  the 
dorsal  surface.  A  broadening  and  a  shortening 
of  this  germ-band  then  takes  place  so  that  the  pos- 
terior end  of  the  embryo  coincides  with  the  posterior 
end  of  the  egg  and  the  edges  of  the  embryo  grow 
laterally  around  the  egg  until  they  meet  in  the 
median  dorsal  line.  Meanwhile  various  changes 
have  taken  place  within  the  embryo,  among  which 
is  the  formation  of  the  germ  glands  or  ovaries. 

Returning  now  to  a  consideration  of  the  germ  cells, 
we  shall  see  that  it  is  possible  to  trace  the  descendants 
of  the  primordial  germ  cell  with  comparative  ease. 
This  cell  divides  by  mitosis,  forming  two  oogonia 
approximately  equal  in  size  (Fig.  18).  These  two 
then  produce  four  oogonia  of  the  second  order 
(Fig.  19),  and  these  in  turn  increase  by  mitosis, 
forming  eight  oogonia  of  the  third  order  (Fig.  20). 
When  this  stage  is  reached  a  period  sets  in  during 
which  the  oogonia  do  not  divide,  but  are  apparently 
passively  carried  about  by  the  somatic  tissues  as 
shown  in  Fig.  21,  where  they  occupy  a  position 
near  the  end  of  the  tail  fold. 

One  of  the  most  satisfactory  conditions  in  the 
keimbahn  of  Miastor  is  the  comparatively  large 
size  and  peculiar  structure  of  the  primordial  germ 
cells  leaving  in  the  mind  of  the  observer  no  doubt 
as  to  the  identity  of  the  cells  concerned.  Through- 
out the  entire  embryonic  development  of  this  insect 
the  germ  cells  are  considerably  larger  than  any  of 
the  somatic  cells.     The  nuclei  are  correspondingly 


THE   P^DOGENETIC   FLY,   MIASTOR       61 

large  and  are  characterized  by  the  possession  of  a 
number  of  spherical  chromatin  granules  which 
are    evenly    scattered    about   in   the    nuclear    sap. 


Fig.  18.  —  Miastor  nmericana.  Longitudinal  section  through  an  egg 
with  two  oogonia  (oogi).  6 c  =  blastoderm  nucleus;  c/i==  chro- 
matin remains. 

Fig.  19.  —  Miastor  americana.  Longitudinal  section  through  an  egg 
with  four  oogonia  {oog^. 


62 


GERM-CELL   CYCLE   IN   ANIMALS 


Even  under  the  lower  powers  of  the  compound  micro- 
scope the  germ  cells  stand  out  with  great  distinct- 
ness and  could  not  possibly  be  confused  with  any 
other  cells  in  the  embryo. 


c  R 


20 

Fig.  20. — Miastor  americana.  Longitudinal  section  through  an  egg 
with  eight  oogonia  {ooqz).     cR  —  chromatin  remains. 

Fig.  21.  —  Miastor  americana.  Sagittal  section  through  embryo  show- 
ing oogonia  {oog-i)  near  end  of  tail  fold. 


THE  P.EDOGENETIC  FLY,  MIASTOR       63 

During  the  shortening  and  broadening  of  the  germ 
band  the  group  of  eight  oogonia  of  the  third  order 
becomes  separated  into  two  rows  of  four  each  —  one 
row  on  either  side  of  the  body  in  the  region  of  the 
eleventh  segment  (Fig.  22).  Each  group  of  four 
oogonia  then  becomes  surrounded  by  a  layer  of 
mesoderm  cells  and  forms  a  more  or  less  spherical 
body  which  may  now  be  called  an  ovary  (Fig.  23). 
Soon  after  this  occurs,  the  oogonia  begin  to  divide 
again  (Fig.  23,  a)  and  by  successive  mitoses  there 
are  formed  oogonia  of  the  fourth,  fifth  (Fig.  24),  and 
sixth  orders.  This  completes  the  number  of  oogonia, 
which  is  typically  thirty-two  in  each  ovary,  and 
provides  us  with  the  only  case  thus  far  on  record 
where  the  number  of  oogonial  divisions  during  the 
multiplication  period  in  the  history  of  the  germ 
cells  is  known  (Fig.  26). 

There  are  then  six  of  these  oogonial  divisions 
between  the  formation  of  the  single  primordial  germ 
cell  and  the  production  of  the  complete  number  of 
oogonia  in  the  two  ovaries.  Some  of  the  oogonia  of 
the  fifth  order  may  be  prevented  from  dividing,  in 
which  case  of  course  there  are  less  than  thirty-two 
germ  cells  in  each  ovary.  And  not  all  of  the  oogonia 
in  the  ovary  succeed  in  developing  into  oocytes  and 
larvae,  since  a  struggle  for  supremacy  takes  place 
among  the  germ  cells  resulting  in  the  survival  of  only 
a  few  offspring,  as  may  be  determined  by  the  fact, 
already  referred  to,  that  one  larva  gives  rise  as  a 
rule  to  onlv  from  five  to  seventeen  daughter  larvae. 
Each  oogonium  that  succeeds  in  developing  becomes 


64 


GERM-CELL   CYCLE  IN  ANIMALS 


22 


00  c 


24 


Fig.  22. — Miastor  americana. 

of  embryo  showing  oogonia 
Fig.  23. — Miastor  americana. 

one  dividing  by  mitosis  (o). 
Fig.  24.  —  Miastor    americana. 

{o6(ji).     m  =  mesoderm. 
Fig.  25. — Miastor   americana. 

(n.c). 


Frontal  section  through  posterior  end 
Koogs)  formmg  two  rows  of  four  each. 
Ovary  containing  sixteen  oogonia  (odgi), 

m  =  mesoderm. 

Ovary  containing  thirty-two   oogonia 

Young  oocyte  {ooc)  with  nurse   cells 


to 


Cfr,- 

PP' 

I^pl ... 

st.c 

PP' ^^ 

PP' 


EI 


..oh 


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,pt. 


■  eithf 


.P5«~... 


ti-Ott 


063*..., 
00^ 


OX© 


.©X©Y©, 


0IO, 


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00/-. 
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.^f4ljV 


_-Ja?-Oo 


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^^(bO_oo^oni_flL  tO_>nV"tt  jAjl. 


p.^. 


Fig,  26.  —  Miastor  americana.  Diagram  illustrating  origin  and  history 
of  germ  cells  from  one  generation  to  the  next.  d.n  =  cleavage 
nucleus  ;  ex.chr  =  extruded  chromatin  ;  oog  =  oogonia  ;  p.h  =  polar 
body;  p. y. c  =  primordial  germ  cell;  p.o=  primary  oocyte;  p.pl  = 
polar  plasm,     st.c  =  stem  cell.  (^*'^) 

F 


66    GERM- CELL  CYCLE  IN  ANIMALS 

provided  with  a  group  of  about  twenty-four  meso- 
derm cells  which  form  a  syncytium  at  the  anterior 
end  and  may  be  called  nurse  cells  (Fig.  25,  n.c),  since 
they  furnish  food  material  to  the  oocyte.  Another 
group  of  mesoderm  cells  forms  a  cellular  layer  about 
the  oocyte  and  nurse  cells,  and  thus  constitutes  a 
follicular  epithelium.  At  this  stage  the  oocytes 
break  away  from  the  ovary  and  become  distributed 
in  various  parts  of  the  body  of  the  mother-larva. 

Several    facts    regarding    the    germ-cell    cycle    of 
Miastor    deserve    special    emphasis :     (1)  There    is 
no  stage  in  the  entire  keimbahn  when  the  germ  cells 
cannot  be  distinguished  without  the  least  difficulty ; 
(2)  the  number  of  oogonial  divisions  has  been  defi- 
nitely established,  and  so  it  is  no  longer  necessary  to 
make   the   general    statement   that   the   germ   cells 
pass  through  7i  divisions  during  the  period  of  multi- 
plication, since  here  7i  is  undoubtedly  six;    (3)  the 
descendants   of   the  primordial   germ  cell   are   only 
germ  cells,  i.e.,  the  primordial   germ  cell  does    not 
give  rise  to  both  oogonia  and  nurse  cells  as  seems  to 
be  the  case  in  most  other  insects;    (4)   chromatin- 
diminution  processes  take  place  during  the  mitotic 
divisions  of  the  nuclei  from  the  four-  to  the  eight- 
cell    stage    and    form  the  eight-  to  the    fifteen-cell 
stage  of  such  a  nature  that  all  of  the  cells  in  the 
embryo  finally  are  deprived  of  part  of  their  chromatin 
with  the  exception  of  the  primordial  germ  cell  which 
retains    the    complete    amount    of    this    substance; 
(5)  the  primordial  germ  cell  is  estabhshed  at  the 
eight-cell  stage  and  is  the  first  complete  cell  formed  in 


THE  P^DOGENETIC   FLY,   MIASTOR        67 

embryonic  development ;  and  (6)  the  contents  of 
the  primordial  germ  cell  consist  of  the  nucleus 
with  undiminished  chromatin  and  of  all  of  the  pole- 
plasm  and  apparently  no  other  part  of  the  egg  sub- 
stance. 

The  fact  that  only  the  primordial  germ  cell  re- 
ceives a  complete  amount  of  chromatin  is  of  particu- 
lar interest,  since  a  similar  condition  has  long  been 
known  in  the  case  of  Ascaris  as  we  shall  see  later. 
It  may  also  be  noted  in  this  place  that  the  cyto- 
plasmic substance  in  the  primordial  germ  cell  may 
be  recognized  as  the  pole-plasm  in  the  growing 
oocyte.  Attempts  have  been  made  to  determine  the 
origin  of  this  pole-plasm,  but  so  far  without  success. 
It  may  be  distinguished  from  the  rest  of  the  Qgg  con- 
tents by  its  position  at  the  posterior  end  and  because 
of  its  affinity  for  certain  dyes.  It  appears  shortly 
before  the  maturation  division  is  initiated,  but  no 
transition  stages  have  been  discovered  —  it  has  been 
either  present  or  entirely  absent  in  the  preparations 
thus  far  studied.  If  we  consider  the  history  of  this 
substance  from  the  formation  of  the  primordial 
germ  cell  to  the  growth  period  of  the  oocytes  pro- 
duced by  this  primordial  germ  cell,  we  may  conclude 
that  at  the  time  the  multiplication  period  ends  the 
pole-plasm  has  become  equally  distributed  among  the 
sixty-four  oogonia.  Then  ensues  the  growth  period 
during  which  the  pole-plasm  cannot  be  distinguished. 
Later,  however,  just  before  maturation,  pole-j^lasm 
substance  reappears  which  is  equal  in  amount  to 
that  contained  in  the  primordial  germ  cell  of  the 


68  GERM-CELL  CYCLE  IN  ANIMALS 

preceding  generation  or  to  that  contained  in  all  of 
the  sixty-four  oogonia  which  descended  from  that 
primordial  germ  cell.  That  is,  the  pole-plasm  of 
the  oocyte  under  discussion  has  in  some  way  increased 
until  its  mass  is  sixty-four  times  as  great  as  that  of 
the  oogonium  before  the  growth  period  began.  How 
this  increase  has  taken  place  can  only  be  conjectured. 
The  pole-plasm  in  the  oogonium  may  have  produced 
new  material  of  its  own  kind  either  by  the  division 
of  its  constituent  particles  or  by  the  influence  of  its 
presence.  In  any  case  a  localization  of  this  substance 
occurs  at  the  posterior  end  of  the  egg  just  before 
maturation.  Therefore,  although  we  can  follow  the 
germ  cells  in  Miastor  throughout  their  entire  cycle 
without  difficulty,  there  are  certain  problems,  such 
as  the  history  of  the  pole-plasm  during  the  growth 
period  of  the  oocytes,  which  still  remain  unsolved. 


CHAPTER  IV 

THE     SEGREGATION     OF     THE     GERM     CELLS     IN 
PORIFERA,    CCELENTERATA,   AND    VERTEBRATA 

The  history  of  the  germ  cells  has  not  been  seriously 
investigated  in  a  number  of  groups  of  animals,  but,  as 
will  be  demonstrated  in  Chapters  V  and  VI,  there 
are  many  species  belonging  to  widely  separated 
groups  in  the  animal  series  in  which  the  germ-cell 
cycle  is  almost  as  well  known  as  in  Miastor.  On 
the  other  hand,  the  three  phyla  to  be  discussed  in 
this  chapter  have  been  carefully  studied  for  many 
years,  but  an  early  segregation  of  germ  cells  has  not 
yet  been  established  in  them  to  the  satisfaction  of  a 
majority  of  investigators.  It  seems  strange  because 
of  the  uncertainty  of  the  morphological  continuity 
of  the  germ  cells  in  these  animals  that  one  of  these 
groups,  the  Cgelenterata,  should  have  furnished 
the  material  upon  which  Weismann  based  his  elabo- 
ration of  the  germ-plasm  theory. 

1.     PORIFERA 

Sponges  reproduce  asexually  by  budding  and  by 
the  formation  of  gemmules,  and  sexually  by  means  of 
ova  and  spermatozoa.  Budding  occurs  in  almost  all 
sponges.  In  most  cases  the  buds  remain  attached 
to  the  parent  (continuous  budding) ;    but  in  some 

69 


70  GERM-CELL   CYCLE   IN  ANIMALS 

species  the  buds  become  free  (discontinuous  bud- 
ding) . 

Gemmules  are  groups  of  cells  (statocytes)  which 
occur  at  certain  times  of  the  year  in  the  bodies  of 
fresh- water  sponges  and  in  many  marine  species. 
These  gemmules  acquire  a  resistant  covering  and 
serve  to  preserve  the  race  during  the  winter  in  the 
north  or  the  dry  season  in  the  south.  The  peculiar 
"  budding"  observed  in  Tetkya  by  Deso  (1879,  1880) 
may  be  a  sort  of  gemmule  formation  (see  p.  76). 

The  eggs  and  spermatozoa  are  situated  in  the 
middle  layer  (so-called  mesoderm)  and  in  most 
cases  seem  to  become  ripe  at  different  times  in  the 
same  sponge.  Fertilization  is  apparently  similar 
to  this  process  in  other  Metazoa.  The  fertilized 
ovum  is  holoblastic;  the  free-swimming  ciliated 
larva  becomes  fixed,  and  then  metamorphoses  into 
a  young  sponge. 

The  body  wall  of  the  sponge  consists  of  two  distinct 
layers,  an  outer  dermal  layer  and  an  inner  gastral 
layer,  and  an  intermediate  jelly-like  stratum  con- 
taining ameboid  wandering  cells.  The  various  sorts 
of  cells  in  these  lavers  are  indicated  in  the  table  on 
page  71  (from  Minchin,  1900,  p.  62). 

The  reproductive  cells  lie  in  the  jelly-Kke  middle 
layer,  but  all  of  the  cells  in  this  layer  are  not  repro- 
ductive. 

The  origin  of  the  archeocytes  from  which  the  re- 
productive cells  arise  can  easily  be  pointed  out  in 
the  comparatively  simple  development  of  Clathrina 
hlanca  (Minchin,   1900).     In  this  species  a  ciliated 


PORIFERA,   CCELENTERATA,   VERTEBRATA     71 


Table  of  the  Various  Classes  of  Cells  in  Sponges 


Dermal  Layer 


I.  Epithelial  stratum 


II.  Porocytes 


III.  Skeletogenous 
stratum 


Gastral  Layer     IV.    Gastral  epithelium 


Archaeocytes 
(primordial 
cells) 


V.  Amebocytes  (wan- 
dering cells) 


2. 

3. 
4. 

^    6. 

7. 

8. 

9. 
10. 
11. 
12. 
13. 
U. 


VI.    Tokocytes  (repro-  -j  ,  ^ 
ductive  cells) 


Pinacocytes 

(epithelial  cells) 
Myocytes 

(contractile 
cells) 
Gland  cells 
Spongoblasts 
Pore  cells 
Scleroblasts 
CoUencytes 

(stellate  cells) 
Desmac3"tes 

(fiber  cells) 
Cvstencvtes 

(bladder  cells) 
Choanocytes 

(collar  cells) 
Phagocytes 

(ingestive  cells) 
Trophocytes 

(nutritive  cells) 
Thesocytes 

(storage  cells) 
Statocytes 

(gemmule  cells) 
Gonocytes 

(sexual  cells) 


blastula-like  larva  is  formed  (Fig.  27,  A).  At  the 
posterior  pole  two  blastomeres  (posterior  granu- 
lar cells,  p.g.c.)  remain  undifferentiated ;  they  are 
much  larger  than  the  other  cells,  are  granular,  and 
possess  vesicular  nuclei.  The  larva  becomes  fixed 
by  the  anterior  pole,  and  during  the  metamorphosis 
that  then  takes  place,  the  two  posterior  granular 
cells,  the  archeocytes,  multiply  rapidly,  forming  a 
large  number  of  minute  cells  which  resemble  certain 
leucocytes.     These  are  known  as  amebocytes.     By 


72 


GERM-CELL   CYCLE   IN  ANIMALS 


the  fourth  day  the  amebocytes   become   separated 
into  wandering  cells  or  their  derivatives  and  repro- 
ductive cells  or  tokocytes  as  indicated  in  the  table. 
The  primordial  archeocytes  do  not  always  occur 
in  the  Clathrinidse  as  in  Clathrina  hlanca.     In  some 


Fig.  27.  —  A.  Clathrina  hlanca.  Blastula  stage  showing  posterior  gran- 
ular cells  (p.g.c).  (From  Minchin,  1900.)  B.  Oogonium  of  a 
sponge  containing  inclusions  in  the  cytoplasm.  {From  Jorgensen, 
1909.)  C.  Two  oogonia  in  the  ectoderm  of  Hydra  fusca,  each  with 
a  cytoplasmic  inclusion.     {From  Downing,  1909.) 


species  there  is  only  one;  in  others  four  or  more 
appear ;  and  sometimes  they  are  entirely  absent. 
This  last  condition  results  from  the  formation  of 
amebocytes  before  the  fixation  of  the  larva.  In 
many  other  sponges  the  archeocytes  migrate  in  at 
the  posterior  pole  and  partially  or  entirely  fill  up 
the  segmentation  cavity.  Comparatively  little  is 
known  about  the  embrvologv  of  Hexactinellida  and 


PORIFERA,   CCELENTERATA,   VERTEBRATA     73 

Demospongiae,  and  few  observations  have  been 
made  upon  their  archeocytes.  These  archeocytes 
are  of  the  greatest  importance  since  they  give  rise 
to  the  amebocy tes  and  tokocy tes  (reproductive  cells) . 
According  to  Weltner  (1907)  both  amebocy  tes  and 
tokocytes  are  only  physiological  states  of  one  and 
the  same  kind  of  cell.  Many  authors  have  em- 
phasized the  importance  of  the  amebocytes,  such  as 
Gorich  (1904),  who  maintains  that  this  class  of  cells 
gives  rise  not  only  to  the  gonocytes,  statocytes, 
and  trophocytes,  but  also  to  certain  pinacocytes. 
Weltner  (1907)  goes  further  than  this  when  he  states 
from  studies  upon  the  fresh-water  sponge  that  the 
sponge  could  not  exist  without  amebocytes. 

The  earlier  investigators  almost  invariably  con- 
sidered the  germ  cells  as  mesodermal  in  origin. 
Lieberklihn  (1856)  discovered  the  eggs  in  Spongilla 
and  later  (1859)  in  Sycandra  raphanus.  Sponge 
eggs  were  also  observed  by  Kolliker  (1864).  Haeckel 
(1872)  thought  that  the  eggs  were  derived  from  the 
flagellated  cells  of  the  gastral  epithelium.  Schulze 
(1875),  on  the  contrary,  maintains  that  they  lie 
deep  in  the  so-called  mesoderm;  and  Fiedler  (1888) 
concludes  that  in  Spongilla  only  certain  cells  of  the 
middle  layer  may  become  germ  cells. 

Maas  (1893)  distinguished  two  sorts  of  cells  in  the 
middle  layer ;  one  characterized  by  uniform,  fine- 
granuled  cytoplasm  and  an  oval  nucleus  containing 
a  very  fine  net-work  of  chromatin ;  the  other  filled 
with  coarse-granuled  cytoplasm  and  a  spherical  nu- 
cleus containing   a    deeply  staining    nucleolus    and 


74  GERM-CELL  CYCLE   IN  ANIMALS 

chromatin  aggregated  into  large  masses.  Only 
from  the  latter  do  the  sex  cells  arise.  These  two 
kinds  of  cells  could  be  distinguished  in  larval  stages 
and  the  early  separation  of  germ  cells  from  somatic 
cells  was  pointed  out.  Maas,  however,  does  not 
insist  that  there  is  here  a  demonstrated  continuity 
of  germ  cells,  since  the  cells  which  become  sex-cells 
are  separated  from  the  egg  by  a  long  series  of  genera- 
tions. 

The  recent  investigations  of  Jorgensen  (1910) 
on  Sycon  raphanus  and  S.  setosa  have  added  consider- 
ably to  our  knowledge  of  the  origin,  structure,  and 
early  history  of  the  germ  cells  of  sponges.  Jorgensen 
does  not  agree  with  Maas  (1893)  regarding  the  early 
segregation  of  the  germ  cells  from  somatic  cells, 
but  finds  no  particular  difference  between  so-called 
mesoderm  cells  and  wandering  or  egg  cells.  It  is 
worthy  of  note,  however,  that  the  youngest  recog- 
nizable oogonia  were  found  to  contain  several  distinct 
bodies  in  their  cytoplasm  (Fig.  27,  B). 

The  method  of  formation  of  the  gemmules  has 
engaged  the  attention  of  many  investigators,  but 
several  important  points  concerning  it  are  still  in 
doubt.  Gemmule  formation  is  of  particular  interest 
since  the  cells  (amebocytes),  which  by  most  authori- 
ties are  said  to  give  rise  to  the  germ  cells,  are  also 
considered  the  cells  which  form  the  reproductive 
portion  of  the  gemmules.  At  least  four  views  have 
been  held  concerning  the  origin  of  the  gemmule 
cells  :  (1)  Carter  (1849)  believed  that  the  gemmule 
is  derived  from  a  single  cell,  the  "ovi-bearing  cell"; 


PORIFERA,   CCELENTERATA,   VERTEBRATA     75 

(2)  Goette  (1886)  maintains  that  the  gemmule  con- 
sists of  cells  from  several  germ  layers ;  (3)  Carter 
believed  at  one  time  that  the  gemmule  was  made 
up  of  only  one  kind  of  cell ;  and  (4)  several 
authors  (Marshall,  1884 ;  Wierzejski,  1886 ;  Zykoff, 
1892;  Weltner,  1892)  believe  that  a  number  of 
cells  belonging  to  several  classes  are  concerned  in 
the  origin  of  the  gemmule. 

Evans  (1900)  has  described  in  detail  the  formation 
and  structure  of  the  gemmules  of  Ephydatia  hlemhin- 
gia.  In  this  species  the  first  sign  of  the  formation  of  a 
gemmule  is  the  presence  of  "single  cells  or  groups 
of  cells  scattered  about  chiefly  in  the  dermal  mem- 
brane ;  the  strands  of  tissue  which  support  the  dermal 
membrane ;  and  in  the  tissues  situated  immediately 
below  the  subdermal  cavity"  (p.  89).  No  mitotic 
figures  were  discovered  in  these  cells  and  conse- 
quently the  reproductive  part  of  the  gemmule  is 
probably  not  derived  from  one  mother-cell.  These 
cells  wander  "through  the  dermal  membrane,  and 
strands  of  tissue  which  support  the  membrane,  and 
become  aggregated  in  groups  situated  either  deep 
in  the  tissues  of  the  sponge  or  even  in  the  strands  of 
tissue  above  mentioned." 

Whether  the  reproductive  cells  of  the  gemmule 
arise  from  a  single  cell  by  proliferation  or  represent 
an  aggregation  without  a  common  origin  is  still 
unsettled,  but  the  latter  view  is  held  by  most  in- 
vestigators. If  they  do  arise  from  a  single  cell,  as 
H.  V.  Wilson  (1902)  admits  is  a  possibility,  the 
gemmule  formation  may  be  considered   a   kind   of 


76  GERM-CELL   CYCLE   IN  ANIMALS 

parthenogenesis.  If,  on  the  other  hand,  the  re- 
productive cells  of  the  gemmule  are  of  multiple 
origin,  they  may  either  be  looked  upon  as  true  germ 
cells  which  form  a  group  physiologically  equivalent 
to  the  morula  stage  in  the  development  of  an  egg, 
or  as  a  collection  of  regenerative  cells  capable  of 
producing  a  new  individual. 

In  this  connection  should  be  mentioned  the  bud- 
ding of  Tethya  (Deso,  1879-1880)  which  develops 
from  a  group  of  amebocytes  (Maas,  1910)  and  the 
gemmules  of  Tedania  and  Esperella  (Wilson,  1902) 
and  of  hexactinellids  (Ijima)  which  become  ciliated 
larvae.  Wilson  has  shown  "  that  silicious  sponges, 
when  kept  in  confinement  under  proper  conditions 
degenerate  in  such  a  manner  that  while  the  bulk 
of  the  sponge  dies,  the  cells  in  certain  regions  become 
aggregated  to  form  lumps  of  undifferentiated  tissue. 
Such  lumps  or  plasmodial  masses,  which  may  be 
exceedingly  abundant,  are  often  of  a  rounded  shape 
resembling  gemmules,  more  especially  the  simpler 
gemmules  of  marine  sponges  (Chalina,  e.g.),  and 
were  shown  to  possess  in  at  least  one  form  (Stylo- 
tella)  full  regenerative  power.  When  isolated  they 
grow  and  differentiate,  producing  perfect  sponges  " 
(1907,  p.  295).  These  "lumps  of  undifferentiated 
tissue"  have  also  been  noted  by  F.  E.  Schulze 
(1904)  and  recognized  as  probably  reproductive; 
they  have  been  named  by  this  author,  "  sorites,"  and 
have  been  called  by  several  authors  "artificial 
gemmules."  The  process  involved  in  their  forma- 
tion   is    termed    "regressive    differentiation."     The 


PORIFERA,   CCELENTERATA,   VERTEBRATA     77 

undifferentiated  tissue  of  which  they  are  composed, 
undoubtedly  consists  largely,  if  not  entirely,  of 
amebocytes  (Weltner,  1907).  These  amebocytes 
are,  however,  of  heterogeneous  origin  (Maas,  1910), 
since  some  of  them  represent  transformed  pore 
cells,  whereas  the  rest  are  wandering  cells. 

Even  more  interesting  than  these  reproductive 
bodies  are  the  artificial  plasmodia  produced  by  Wil- 
son (1907,  1911)  in  Microciona,  Lissodendoryx,  and 
Stylotella  and  by  Muller  (1911)  in  the  Spongillidae. 
The  method  and  results  from  a  study  of  Microciona 
as  stated  by  Wilson  (1911)  are  briefly  as  follows. 
Branched  specimens  are  cut  up  and  strained  into  a 
dish  of  water  through  fine  bolting  cloth.  The 
cells,  which  are  dissociated  in  this  way,  "settle 
down  on  the  bottom  of  the  dish  like  a  fine  sediment." 
Three  classes  of  cells  are  present :  (1)  *' the  most  con- 
spicuous and  abundant"  are  unspecialized  granular 
"ameboid  cells  of  the  sponge  parenchyma  (amoebo- 
cytes) " ;  (2)  "a  great  abundance  of  partially 
transformed  collar  cells";  and  (3)  "more  or  less 
spheroidal  cells  ranging  from  the  size  of  the  granular 
cells  down  to  much  smaller  ones." 

"Fusion  of  the  granular  cells  begins  imme- 
diately and  in  a  few  minutes'  time  most  of  these 
have  united  to  form  conglomerate  masses  which  at 
the  surface  display  both  blunt  and  elongated  pseu- 
dopodia.  These  masses  (plasmodia)  soon  begin  to 
incorporate  the  neighboring  collar  and  hyaline  cells." 
"The  small  conglomerate  masses  .  .  .  early  begin  to 
fuse  with  one  another,"  and  if  the  tissue  is  strewn 


78  GERM-CELL  CYCLE   IN   ANIMALS 

sparsely  over  a  slide,  in  the  course  of  a  week  it  will 
be  found  that  the  slide  is  covered  with  a  thin  in- 
crusting  sponge  provided  with  pores,  oscula,  canals, 
and  flagellated  chambers."  Many,  at  the  end  of 
two  months,  had  "  developed  reproductive  bodies 
(eggs  or  asexual  embryos?)  ..."  Whether  these 
reproductive  bodies  arose  from  eggs  or  masses 
of  cells  was  not  determined.  "  When  the  plasmodia 
have  metamorphosed  and  the  canals  and  chambers 
have  developed,  the  skeleton  makes  its  appearance." 

Experiments  with  Lissodendoryx  and  Stylotella 
were  not  quite  so  successful,  but  plasmodial  masses 
were  formed  in  every  case.  Further  experiments 
proved  that  "  when  the  dissociated  cells  of  these 
two  species  [Microciona  and  Lissodendoryx]  are 
intermingled,  they  do  not  fuse  with  one  another, 
but  fusion  goes  on  between  the  cells  and  cell  masses 
of  one  and  the  same  species."  A  similar  result  was 
obtained  by  intermingling  dissociated  cells  of  Micro- 
ciona and  Stylotella. 

Discussion  and  Summary.  The  foregoing  ac- 
count of  the  origin  of  the  germ  cells  in  sponges 
shows  conclusively  that  these  cells  arise  in  the  so- 
called  mesoderm  from  wandering  cells  (amebocytes) 
and  that  amebocytes  are  descended  from  archaeo- 
cytes  which  may  be  distinguished  in  certain  cases 
very  early  in  embryological  development  (Fig.  27,  A, 
p.g.c).  Oogonia  and  spermatogonia  have  not  been 
recognized  by  most  investigators  except  in  the  adult, 
but  Maas  (1893)  has  observed  them  in  the  planula. 
Jorgensen  (1910),  who  has  made  the    most    careful 


PORIFERA,   CCELENTERATA,   VERTEBRATA     79 

study  of  the  development  of  the  oogonia,  states 
that  the  youngest  recognizable  oogonia  lie  in  the 
mesoderm,  and  his  figure  (Fig.  27,  B)  shows  that 
they  may  be  distinguished  from  neighboring  cells  by 
certain  characteristics,  among  which  is  the  presence 
of  a  darkly  staining  inclusion.  In  the  adult  sponge 
the  amebocytes  from  which  the  oogonia  and  sperma- 
togonia arise  occur  in  the  middle  layer  of  all  regions  of 
the  body,  but,  as  pointed  out  by  Korschelt  and 
Heider  (1903),  the  oogonia  and  spermatogonia  may 
develop  in  only  certain  definite  regions  {Plakina 
monoloplia),  or  in  groups  (Aphysilla  violacea)  which 
contain  a  more  or  less  definite  number  of  cells  and 
occupy  a  similar  position  in  each  individual  (Eu- 
spongia).  Such  an  aggregation  is  the  most  primitive 
form  of  ovary. 

Some  of  the  amebocytes  of  the  sponge  are  un- 
doubtedly germ  cells  (tokocytes)  and  are  able  to 
develop  into  oogonia  or  spermatogonia,  or  to  form 
aggregations  (gemmules,  "  artificial  gemmules,"  '*  so- 
rites," etc.)  which  can  "regenerate"  an  entire  sponge, 
but  whether  the  amebocytes  that  produce  oogonia 
and  spermatogonia  are  the  same  as  the  reproductive 
cells  of  the  gemmules,  the  regenerative  cells  of  the 
"  artificial  gemmules,"  and  amebocytes  which  form 
the  buds  in  Tethya  is  still  uncertain.  It  seems 
probable  that  they  are  all  alike  potentially  but 
develop  differently  because  of  the  effects  of  different 
environmental  factors.  The  distribution  of  ame- 
bocytes with  reproductive  powers  throughout  the 
entire  sponge-body  accounts  for  the  great  regenera- 


80  GERM-CELL   CYCLE   IN  ANIMALS 

tive  ability  of  these  animals  and  must  also  account 
for  the  development  of  plasmodia  formed  by  dis- 
sociated cells  (Wilson,  1911;  Muller,  1911)  into 
adult  sponges  with  all  specific  characteristics  in- 
cluding reproductive  bodies. 

It  therefore  seems  possible  that  there  may  exist 
in  the  sponges  a  continuity  of  the  germ-plasm  and 
that  the  germ-cell  material  is  distributed  among 
thousands  of  cells  (tokocytes,  see  Table,  p.  71) 
which  are  derived  from  archseocytes,  and  that  under 
proper  conditions  these  tokocytes  may  produce 
oogonia  or  spermatogonia,  or  may  aggregate  to 
form  gemmules  or  regenerative  bodies.  This  wide 
distribution  of  the  germ  cells  is  what  might  be 
expected  in  such  lowly  organized  animals.  Figure 
28  shows  the  probable  history  of  the  germ  cells  in 
the  PoRiFERA  from  one  generation  to  the  next. 

2.     CCELENTERATA 

The  origin  of  the  germ  cells  in  the  Ccelenterata 
has  been  a  much  debated  subject  among  zoologists 
for  three-quarters  of  a  century.  As  early  as  1843  van 
Beneden  undertook  to  determine  the  germ  layer 
from  which  the  germ  cells  arise  and  concluded  that 
the  ova  originate  in  the  entoderm  and  that  the 
spermatozoa  come  from  the  ectoderm.  F.  E. 
Schulze  (1871)  claims  that  in  Cordylophora  both 
the  ova  and  spermatozoa  are  of  ectodermal  origin. 
Kleinenberg  (1872),  working  on  Hydra,  announced 
that  the  germ  cells  are  interstitial  in  origin  and, 
since  the  interstitial  cells  arise  from  the  ectoderm, 


PORIFERA,   CGELENTERATA,    VERTEBRATA     81 


Statocytes 
{Gemmule  cells) 


Archeocytes 


Fig.  28. 


Spermatozoon 


■  Diagram  illustrating  the  probable  history  of  the  germ  cells 

in  sponges  from  one  generation  to  the  next. 
G 


82  GERM-CELL   CYCLE   IN  ANIMALS 

are  therefore  also  ectodermal.  Van  Beneden  (1874), 
from  investigations  on  Hydractinia,  Clava,  and  Cam- 
PANULARiD^,  confirms  his  earlier  results  and  again 
maintains  that  the  ova  arise  in  the  entoderm.  The 
brothers  Hertwig  (1878)  decided  that  the  germ  cells 
of  Hydromedus^  arise  from  the  ectoderm  and  those 
of  the  ScYPHOMEDUS^  and  Anthozoa  from  the 
entoderm.  In  a  second  paper,  Kleinenberg  (1881) 
reports  the  ova  of  Eudendrium  as  of  ectodermal 
origin.  Varenne  (1882)  maintains  that  both  the 
ova  and  the  spermatozoa  of  half  a  dozen  species 
examined  arise  from  entoderm  cells  of  the  young 
blastostyle  before  the  appearance  of  medusa  buds. 
The  results  of  Weismann's  extended  studies  were 
published  in  a  monograph  (1883),  and  later  (1884)  a 
brief  general  account  appeared. 

From  this  time  until  the  present  day  almost  every 
year  has  witnessed  one  or  more  contributions  to  the 
subject  of  the  origin  of  the  germ  cells  in  ccelenterates, 
and  a  perusal  of  this  mass  of  literature  shows  that 
the  problem  is  not  yet  solved. 

Hydra.  The  fresh-water  polyp.  Hydra,  has  been 
employed  for  germ-cell  investigations  more  often 
than  any  other  coelenterate,  and  a  number  of  de- 
tailed papers  have  appeared  mthin  the  past  ten 
years  upon  this  genus.  Among  the  earlier  workers 
who  actually  saw  the  ^gg  should  be  mentioned 
Trembley  (1744),  Rosel  V.  Rosenhoff  (1755),  Ehren- 
berg  (1836)  and  Leydig  (1848).  The  processes 
involved  in  oogeneses  were  not  clearly  determined, 
however,   until   Kleinenberg's   classic  investigations 


PORIFERA,   CCELENTERATA,   VERTEBRATA     83 

in  1872,  upon  which  most  of  the  accounts  in  our 
zoological  textbooks  are  still  based.  Kleinenberg's 
researches  were  followed  by  those  of  Korotneff 
(1883),  Nussbaum  (1887),  Schneider  (1890),  and 
Brauer  (1891).  Investigations  of  the  germ  cells  of 
Hydra  then  almost  ceased  until  1904,  when  another 
period  of  activity  in  this  field  began  and  papers 
quickly  followed  one  another  (Guenther,  1904 ; 
Downing,  1905;  Hadzi,  1906;  Hertwig,  R.,  1906; 
Tannreuther,  1908,  1909;  Downing,  1909;  and 
Wager,  1909).  The  following  account  is  based 
chiefly  upon  the  researches  of  Downing  (1905, 
1908,  1909),  Tannreuther  (1908,  1909),  and  Wager 
(1909). 

The  origin  of  the  male  germ  cells  has  been  carefully 
investigated  by  Downing  (1905)  and  Tannreuther 
(1909).  Previous  to  Downing's  researches  all  in- 
vestigators, beginning  with  Kleinenberg  (1872), 
considered  the  sex  cells  as  interstitial  in  origin. 
Downing,  however,  believes  that  germ  cells  and  in- 
terstitial cells  may  be  distinct.  The  sex  cells, 
according  to  this  investigator,  are  distinguished 
"by  their  very  large  nuclei,  extremely  granular, 
and  often  by  the  presence  of  a  Nebenkern  "  (Fig. 
27,  C).  "The  characters  of  the  sex  cells  .  .  . 
seem  constant,  and  my  conclusion  would  be  that  at 
some  stage  of  the  embryonic  development  certain 
cells  are  stamped  with  these  characters  and  that  they 
and  their  progeny  form  the  sex  cells  distinct  through- 
out the  life  of  the  individual  .  .  .  the  germ-plasm  is 
then  continuous  in  Hydra  "  (p.  413).     This  tentative 


84  GERM-CELL   CYCLE   IN   ANIMALS 

opinion  is  expressed  with  more  certainty  in  a  later 
paper  (Downing,  1909),  since  the  "distinctive  charac- 
ter of  the  germ  cell  is  more  marked  in  the  ovary  than  in 
thespermary"  (p.  311).  Tannreuther  (1909),  on 
the  other  hand,  claims  that  the  male  germ  cells  are 
interstitial  in  origin,  and  "  the  progenitors  of  the 
spermatozoa  have  no  special  characters  by  which 
they  can  be  recognized  as  germ  cells." 

The  origin  of  the  eggs  of  Hydra  is  better  known 
than  that  of  the  male  germ  cells.  The  ova  have  by 
most  investigators  been  considered  modified  intersti- 
tial cells.  Downing  (1908,  1909)  disagrees  in  several 
respects  with  the  results  of  Tannreuther  and  Wager. 
His  most  important  difference  is  regarding  the  ques- 
tion of  the  origin  of  the  ova  directly  from  interstitial 
cells  or  from  definite  propagative  cells  that  are  set 
aside  for  reproductive  purposes  at  some  stage  in  the 
animal's  embryonic  development.  He  believes  "  that 
in  the  adult  Hydra  the  oogonia  (and  spermatogonia) 
are  distinctly  differentiated  as  a  self-propagating 
tissue"  (p.  310).  Wager  (1909),  on  the  contrary, 
claims  that  it  is  impossible  to  prove  that  eggs  do 
not  arise  from  ordinary  interstitial  cells;  whereas 
Tannreuther  (1909)  finds  that  the  primitive  ova  can 
be  distinguished  from  interstitial  cells  "by  their 
large  nucleus,  nucleolus,  and  abundance  of  chromatin, 
even  before  the  growth  of  the  ovary  begins"  (p.  205), 
especially  during  the  breeding  season,  and  admits 
that  "If  these  sex  cells  could  be  distinguished  during 
the  budding  season  as  well,  it  would  at  least  suggest 
specificity  of  the  germ  cells  "  (p.  205). 


PORIFERA,    CGELENTERATA,   VERTEBRA  FA     85 

By  far  the  most  important  question  arisin^^  from 
a  study  of  the  origin  of  the  germ  cells  of  Hydra  is 
whether  these  cells  arise  from  ordinary  interstitial 
cells,  as  is  claimed  by  most  investigators,  or  whether 
they  originate  from  cells  that  are  set  aside  for  re- 
productive purposes  at  some  stage  of  development, 
as  Downing  maintains.  If  the  latter  be  true,  "the 
germ-plasm  is  then  continuous  in  Hydra  ^  (Downing, 
1905,  p.  413). 

Wager  (1909)  thinks  the  presence  of  special  prop- 
agation cells  to  be  "extremely  improbable"  and 
Tannreuther  (1909)  does  not  believe  the  known 
facts  warrant  the  view  that  there  is  continuity 
of  the  germ-plasm  in  Hydra.  This  is,  of  course,  a 
matter  that  may  never  be  decided  definitely,  and  at 
least  not  until  some  method  of  distinguishing 
the  primordial  germ  cells,  if  these  be  present,  from 
ordinary  interstitial  and  other  cells,  has  been  found. 
Furthermore,  if  the  germ-plasm  is  continuous,  primor- 
dial germ  cells  must  be  present  in  buds,  in  adults  at 
all  times  of  the  year,  and  in  pieces  of  tissue  that  are 
capable  of  regenerating  sexually  reproductive  adults. 
That  such  primordial  germ  cells  exist  seems  to  me  to 
be  quite  possible. 

Hydrozoa.  Many  Hydrozoa  besides  Hydra  have 
furnished  material  for  germ-cell  studies.  Thus 
Weismann  (1883)  reported  upon  about  forty  species 
belonging  to  a  number  of  different  families.  The 
results  of  the  researches  of  the  various  investigators 
do  not  agree  in  many  instances.  In  order  to  indicate 
the  variety  of  the  opinions  expressed,  the  data  re- 


86  GERM-CELL  CYCLE  IN  ANIMALS 

garding  the  germ  cells  in  the  following  genera  is 
considered  below :  (1)  Eiidendrium,  {%)  Hijdractinia, 
(3)  Pennaria,  and  (4)   Clava, 

EuDENDRiUM.  Fivc  spccies  of  this  genus  have 
been  investigated.  In  E.  racemosum,  according  to 
Weismann  (1883),  the  ova  arise  in  the  ectoderm  and 
the  male  germ  cells  originate  either  from  entoderm 
cells  or  from  ectoderm  cells  that  later  migrate  into  the 
entoderm.  Ischikawa  (1887)  asserts  that  the  germ 
cells  arise  in  the  ectoderm  and  migrate  into  the  en- 
toderm, and  Hargitt  (1904a)  found  ova  in  both 
the  ectoderm  and  entoderm,  but,  since  those  in  the 
entoderm  were  always  the  smaller,  he  concludes  that 
they  may  have  wandered  into  that  layer  from  the 
ectoderm,  though  such  a  migration  was  not  ob- 
served. 

In  E.  capiUare  Hargitt  found  ova  in  the  entoderm 
except  in  one  case  where  they  occurred  in  the  ecto- 
derm. This  author  also  reports  the  female  germ 
cells  of  E.  tenue  and  E.  racemosum  from  the  entoderm 
only.  The  ova  of  the  Eudendrid^  when  first  dis- 
tinguishable "are  slightly  larger  than  the  ordinary 
cells  of  the  surrounding  tissue,  and  differ  also  in 
shape,  being  generally  ovoid  or  spherical  and  with 
comparatively  conspicuous  nuclei.  .  .  .  Growth  at 
this  period  would  seem  to  take  place  in  situ,  through 
the  direct  nutritive  activity  of  the  surrounding  tissue 
cells.  ...  As  growth  continues,  the  ova  become 
more  or  less  amoeboid,  migrating  toward  the  gono- 
phore  region,  where  they  seem  to  aggregate  in  con- 
siderable numbers,  the  presence  of  which  may  act  as  a 


PORIFERA,   CCELENTERATA,   VERTEBRATA     87 

stimulus  from  which  results  the  formation  of  the 
gonophore"   (Hargitt,  1904  a,  pp.  261-262). 

Hydractinia  has  been  investigated  by  van 
Beneden  (1874),  Weismann  (1883),  Bunting  (1894), 
and  Small  wood  (1909).  Weismann  considered  the 
ectoderm  of  the  blastostyle  to  be  the  probable  place 
of  origin  of  the  germ  cells  in  this  genus.  Bunting 
(1894)  was  unable  to  trace  the  ova  to  this  layer, 
although  she  found  them  to  be  quite  abundant  in 
the  entoderm  of  the  blastostyle,  even  before  the  gono- 
phore appeared.  According  to  this  author  the  ova 
apparently  arise  in  the  entoderm  of  the  blasto- 
style, and  "reach  maturity  on  the  outside  wall  of 
the  spadix,  lying  between  the  endoderm  and  the 
inner  layer  of  the  bell  nucleus.  The  spermatozoa 
arise  from  the  inner  layer  of  the  bell  nucleus ;  we 
see  that  they  are,  therefore,  ectodermal  in  origin  " 
(p.  228). 

These  results  are  not  confirmed  by  the  researches 
of  Small  wood  (1909),  who  finds  that  the  eggs  arise 
in  the  entoderm  in  any  region  of  the  polyp,  at  the 
base,  the  side  of  the  polyp,  or  in  the  gonophore. 
They  may  be  distinguished  from  other  entoderm 
cells  by  the  larger  size  of  the  nucleus. 

In  Pennaria  cavolini  the  germ  cells  arise  in  the 
ectoderm,  according  to  Weismann  (1883),  and  this 
conclusion  is  confirmed  for  the  ova  by  Hargitt 
(19046).  In  P.  tiarella  the  germ  cells  are  likewise  of 
ectodermal  origin  (Smallwood,  1899,  Hargitt,  for 
the  ova,  19046).  The  eggs  of  this  species  arise 
in  the   ectoderm  of   the   manubrium    and  grow  by 


88  GERM-CELL   CYCLE  IN  ANIMALS 

engulfing  other  primitive  ova.  Only  six  or  eight, 
rarely  more,  of  the  eggs  survive. 

In  Clava,  according  to  van  Beneden  (1874),  the 
ova  arise  in  the  entoderm.  Weismann  (1883)  was 
not  able  to  determine  whether  they  originated  in 
the  entoderm  or  migrated  into  that  layer  from  the 
ectoderm,  but  he  was  certain  that  the  male  germ  cells 
were  ectodermal.  This  conclusion  regarding  the 
male  germ  cells  was  confirmed  by  Thallowitz  (1885). 
Harm  (1902)  was  able  to  trace  the  primitive  germ 
cells  back  to  a  very  early  stage,  and  could  distinguish 
them  in  even  young  hydranths.  The  oocytes  dif- 
fered from  the  remaining  ectoderm  cells  in  the  pos- 
session of  a  larger  amount  of  cytoplasm,  a  larger 
nucleus  with  a  big  nucleolus,  and  an  ameboid  shape. 

Hargitt  (1906),  working  on  Clava  leptostyla,  comes 
to  conclusions  different  from  those  of  Harm  on  C. 
squamata.  He  says  "that  eggs  probably  never  arise 
in  the  ectoderm  but  always  in  the  entoderm  of  the 
peduncle  of  thegonophore,  or  in  that  of  the  polyp  very 
near  the  base  of  the  gonophore.  .  .  .  Clava,  like 
other  Hydroids,  has  its  breeding  season,  during  which 
the  germ  cells  are  extremely  abundant,  and  at  other 
times  these  cells  are  either  entirely  absent  or  very 
scarce"  (p.  208).  Concerning  the  early  origin 
of  germ  cells  Hargitt  says,  '"  it  may  not  be  im- 
possible that  '  Urkeimzellen '  should  perhaps  exist  in 
undifferentiated  stages,  still  the  probability  is  so 
extremely  remote  as  to  render  doubtful  to  a  degree 
any  but  the  most  thoroughly  substantial  claims  " 
(p.  209). 


PORIFERA,    (XELENTERATA,   VERTEBRA TA     89 

One  more  Hydrozoon  may  be  mentioned  — 
GonothyroBa  loveni  —  since  Wulfert  (lOO'^)  traced 
the  germ  cells  of  this  species  back  to  the  planula 
stage  where  they  arise  from  the  interstitial  cells  of 
the  ectoderm  and  later  undergo  characteristic 
migrations. 

Our  knowledge  of  the  origin  of  the  germ  cells 
in  other  coelenterates  is  very  fragmentary  and  even 
less  decisive  than  that  of  the  Hydrozoa.  For  this 
reason  a  consideration  of  the  subject  is  omitted  here. 

Discussion.  As  in  the  Porifera  we  are  here 
confronted  with  the  question  whether  or  not  there  is 
continuity  of  the  germ-plasm  in  the  Ccelenterata. 
There  is  sufficient  evidence  for  the  belief  that  the 
cells  which  develop  into  germ  cells  are  not  derived 
from  the  ectoderm  or  the  entoderm  but  belong  to  a 
special  sort  of  propagative  cells  which,  are  scattered 
about  among  the  other  cells  throughout  the  body 
and  which  give  rise  to  ova  or  spermatozoa  under 
certain  environmental  conditions  differing  in  the 
different  species.  This  conclusion  is  based  partly 
upon  the  results  of  Downing  (1905,  1908,  1909), 
who  still  holds,  as  stated  in  his  published  papers, 
that  there  is  continuity  of  the  germ-plasm  in  Hydra  ; 
and  upon  the  fact  that  germ  cells  have  been  recog- 
nized in  the  young  hydrant hs  of  Clava  (Harm,  190'-2) 
and  in  the  planula  of  Gonothyroea  (Wulfert,  190^2). 
It  seems  certain  that  more  careful  studies  of  the 
early  stages  of  coelenterates  with  special  regard 
to  the  origin  of  the  germ  cells  and  with  the  use  of 
many  and  varied  stains  would  result  in  the  discovery 


90 


GERM-CELL   CYCLE  IN  ANIMALS 


&H 


Fig.  29.  —  Diagram  to  illustrate  the  phylogenetic 
shifting  back  of  the  origins  of  the  germ  cells  in 
medusoids  and  hydroids.  A  composite  picture. 
A,  branch  of  a  polyp-colony;  P,  poljiJ-head 
with  mouth  (m)  and  tentacles  ;  »S^,  stalk  of  the 
polyp ;  M,  medusoid-bud  with  the  bell  (Gl) ; 
T,  marginal  tentacle;  m,  mouth;  Mst,  ma- 
nubrium ;  GphK,  a  gonoph ore-bud  ;  GH,  gas- 
tric cavity ;  ekt,  ectoderm  ;  ent,  endoderm ; 
st,  supporting  lamella.  The  germ  cells  (kz) 
arise  in  the  medusoid  in  the  ectoderm  of  the 
manubrium  —  first  phyletic  stage — where  they 
also  attain  maturity.  In  the  gonophore-bud 
(GphK)  they  arise  in  the  ectoderm  (kz^),  or 
further  down  in  the  stalk  of  the  polj^)  at  kz'^ 
—  third  phyletic  stage —  or  in  the  ectoderm  of 
the  branch  from  which  the  polj'p  has  arisen, 
at  kz'"  —  fourth  phyletic  stage  of  the  shunting 
of  the  originative  area  of  the  germ  cells.  In 
the  last  two  cases  the  germ  cells  migrate  until 
they  reach  their  primitive  place  of  origination 
in  the  medusoid,  or  in  the  corresponding  layer 
of  the  medusoid  gonophore,  as  may  be  more 
clearly  seen  in  Fig.  30.    (After  Weismann,  1904-) 


of  these  cells  in 
younger  em- 
bryos than  yet 
recorded,  and 
might  even  dis- 
close charac- 
teristics which 
would  enable 
us  to  trace  the 
keimbahn  in 
some  species 
back  into  the 
early  cleavage 
stages. 

In  discussing 
the  germ  cells 
of  coelenterates, 
it  is  necessary 
to  refer  to  the 
work  of  Weis- 
mann who  has 
added  so  much 
to  our  knowl- 
edge of  this 
subject.  Weis- 
mann's  position 
may  best  be 
presented  in  his 
own  words  {The 
Evolution  The- 
ory, Vol.  I,  pp. 
413-415,  1904). 


PORIFERA,    CCELENTERATA,   VERTEBRATA     91 

"In  the  hydroid  polyps  and  their  medusoids  the 
germ-cells  always  arise  in  the  ectoderm ;  in  species 
which  produce  sexual  medusoids  by  budding,  the 
germ  cells  arise  in  the  ectoderm  of  the  manubrium 
of  these  medusoids  (Fig.  29,  M,  kz).  But  in  many 
species  these  sexual  stages  have  degenerated  in  the 
course  of  phylogeny  into  so-called  gonophores, 
that  is,  to  medusoids  which  still  exhibit  more  or  less 
complete  bells,  but  neither  mouth  (m)  nor  marginal 
tentacles  {T)^  and  which  no  longer  break  away 
from  the  colony  to  swim  freely  about,  to  feed  in- 
dependently, and  to  produce  and  ripen  germ-cells. 
The  degeneration  of  the  'gonophores'  often  goes 
even  farther ;  in  many  the  medusoid  bell  is  repre- 
sented only  by  a  thin  layer  of  cells,  and  in  some  even 
this  token  of  descent  from  medusoid  ancestry  is 
absent,  and  they  are  mere  single-layered  closed 
brood-sacs  (Fig.  30,  Gph). 

"The  adherence  of  the  sexual  animal  to  the  hydroid 
colony  has,  however,  made  a  more  rapid  ripening  of 
the  germ-cells  possible,  and  nature  has  taken  advan- 
tage of  this  possibility  in  all  cases  known  to  me,  for 
the  germ-cells  no  longer  arise  in  the  manubrium  of 
the  mature  degenerate  medusoid,  that  is,  of  the 
gonophore,  but  earlier,  before  the  bud  which  becomes 
a  gonophore  possesses  a  manubrium.  The  birth- 
place of  the  germ-cells  is  thus  shifted  back  from  the 
manubrium  of  the  medusoid  to  the  young  gono})hore- 
bud  (Fig.  29,  M,  kz).  The  same  thing  occurs  in 
species  in  which  the  medusoids  are  liberated,  but  live 
only  for  a  short  time,   for  instance,   in   the  genus 


92 


GERM-CELL   CYCLE   IN   ANIMALS 


Fig.  30.  —  Diagram  to  illustrate  the  migra- 
tion of  the  germ  cells  in  hydromedusae 
from  their  remotely  shunted  place  of  origin 
to  their  primitive  place  of  origin  in  the 
gonophore,  in  which  they  attain  to  ma- 
turity. The  state  of  affairs  in  Eudendrium 
is  taken  as  the  basis  of  the  diagram,  mu, 
mouth  ;  ma,  gut-cavity  ;  t,  tentacle  ;  Sta, 
stem;  A,  a  branch  of  the  polyp-colony; 
SP,  lateral  polyp;  Gph,  a  medusoid-bud 
completely  degenerated  into  a  mere  gono- 
phore ;  Ei,  ovum ;  GH,  gastric  cavity ; 
st,  supporting  lamella.  The  originative 
area  of  the  germ  cells  lies  in  the  stem  of 
the  principal  pob-p  at  kz"",  whence  the 
germ  cells  first  migrate  into  the  endo- 
derm  of  the  branch  (A)  at  kz'",  creeping 
within  which  they  reach  kz"  in  the  lat- 
eral polyp  (blastostyle),  finally  reaching 
the  gonophore  {kz)  and  passing  again 
into  the  ectoderm.  (^After  Weismann, 
1904.) 


Podocoryne.  Al- 
though perfect 
medusoids  are 
formed,  these 
have  their  germ- 
cells  fully  devel- 
oped at  the  time 
of  their  liberation 
from  the  hydroid 
colony.  But  in 
species  in  which 
the  medusoid- 
buds  have  really 
degenerated  and 
are  no  longer  lib- 
erated, the  birth- 
place of  the  germ- 
cells  is  shifted 
even  farther  back^ 
and  in  the  first 
place  into  the 
stalk  (SU  hz")  of 
the  polyp  from 
the  gonophore- 
buds.  This  is  the 
case  in  the  genus 
Hydractinia.  In 
the  further  course 
of  the  process  the 
birthplace  of  the 
germ -cells     has 


PORIFERA,    CCELENTERATA,   VERTEBRATA    93 

shifted  as  far  back  as  to  the  branch  from  which 
the  polyp  has  grown  out  (Fig.  29,  A,  kz'")  ;  and 
finally,  in  the  cases  in  which  the  medusoid  has 
degenerated  to  a  mere  brood-sac  (Fig.  30,  Gph), 
even  to  the  generation  of  polyps  immediately 
before,  that  is,  into  the  polyp-stem  from  which  the 
branch  arises  that  bears  the  polyps  producing 
the  gonophore-bud  (Fig.  30,  kz'").  Then  we  find 
the  birthplace  of  the  germ-cells  still  further  back 
(Fig.  30,  kz""),  for  the  egg  and  sperm  cells  arise 
in  the  stem  of  the  principal  polyps  (the  main  stem 
of  the  colony).  The  advantage  of  this  arrangement 
is  easily  seen,  for  the  principal  polyp  is  present 
earlier  than  those  of  the  secondary  branches,  and 
these  again  earlier  than  the  polyp  which  bears  the 
sexual  buds,  and  this,  finally,  earlier  than  the  sexual 
bud  which  it  bears.  Thus  this  shunting  backwards 
of  the  birthplace  of  the  germ-cells  means  an  earlier 
origin  of  the  primordium  (Anlage)  of  the  germ-cells, 
and  consequently  an  earlier  maturing  of  these. 

"  But  none  of  these  germ-cells  come  to  maturity  in 
the  birthplace  to  which  they  have  been  shifted, 
for  they  migrate  independently  from  it  to  the  place 
at  which  they  primitively  arose,  namely,  into  the 
manubrium  of  the  medusoid,  which  is  still  present 
even  when  great  degeneration  has  occurred,  or  even  — 
in  the  most  extreme  cases  of  degeneration  —  into  the 
ectoderm  of  the  brood-sac.  This  is  the  case  in  the 
genus  Eudendrium,  of  which  Fig.  30  gives  a  diagram- 
matic representation. 

"The  most  interesting  feature  of  this  migration  of 


94  GERM-CELL   CYCLE   IN  ANIMALS 

the  germ-cells  is  that  the  cells  invariably  arise  in 
the  ectoderm  {kz""),  then  pierce  through  the  sup- 
porting lamella  {st)  into  the  endoderm  {kz'^^),  and 
then  creep  along  it  to  their  maturing-place.  Once 
there,  they  break  through  again  to  the  outer  layer  of 
cells,  the  ectoderm  {kz),  and  come  to  maturity  {Ei). 
That  they  make  their  way  through  the  endoderm  is 
probably  to  be  explained  by  the  fact  that  they  are 
there  in  direct  proximity  to  the  food-stream  which 
flows  through  the  colony  {GH  =  gastric  cavity), 
and  they  are  thus  more  richly  nourished  there  than 
in  the  ectoderm.  But,  although  this  is  the  case, 
they  never  arise  in  the  endoderm ;  in  no  single 
case  is  the  birthplace  of  the  germ-cells  to  be  found 
in  the  endoderm,  but  always  in  the  ectoderm,  no 
matter  how  far  back  it  may  have  been  shunted. 
Even  when  the  germ-cells  migrate  through  the  en- 
doderm, their  first  recognizable  appearance  is  in- 
variably in  the  ectoderm,  as,  for  instance,  in  Podo- 
coryne  and  Hydr actinia.  The  course  of  affairs  is 
thus  exactly  what  it  would  necessarily  be  if  our 
supposition  were  correct,  that  only  definite  cell- 
generations  —  in  this  case  the  ectoderm-cells  — 
contain  the  complete  germ-plasm.  If  the  endoderm- 
cells  also  contained  germ-plasm  it  would  be  hard 
to  understand  why  the  germ-cells  never  arise  from 
them,  since  their  situation  offers  much  better  con- 
ditions for  their  further  development  than  that  of 
the  ectoderm-cells.  It  would  also  be  hard  to  under- 
stand why  such  a  circuitous  route  was  chosen  as  that 
exhibited  by  the  migration  of  the  young  germ-cells 


PORIFERA,   CCELENTERATA,   VERTEBRATA     05 

into  the  endoderm.  Something  must  he  hickini.^  in 
the  endoderm  that  is  necessary  to  make  a  cell  into  a 
germ-cell :   that  something  is  the  germ-plasm." 

Several  important  contributions  have  appeared 
within  recent  years  which  seem  to  deprive  Weis- 
mann's  contentions  of  much  of  their  importance. 
For  example,  Goette  (1907)  has  found  that  the  germ 
cells  of  many  Hydromedus^  niay  arise  in  the  en- 
toderm or  in  the  ectoderm,  and  that  in  Clava  multi- 
cornis  the  germ  cells  are  transformed  half-entoderm 
cells.  After  a  long  series  of  studies  on  coelenterate 
development  C.  W.  Hargitt  (1911)  has  attacked 
Weismann's  position  in  the  following  words  :  "  That 
there  is  any  such  region  as  may  be  designated  a 
'Keimzone'  or  'Keimstatte'  may  be  at  once  dis- 
missed as  absolutely  without  warrant  as  a  general 
proposition.  Furthermore,  that  the  germ  cells  have 
their  origin  in  the  ectoderm  alone  in  hydromedusse 
may  be  similarly  denied  and  dismissed  as  unworthy 
of  further  inquiry  or  doubt.  And  still  further,  I  am 
thoroughly  convinced  that  the  still  more  recent 
controversy  as  to  the  hypothesis  of  the  'germ-plasm,' 
if  not  as  clearly  a  delusion  as  the  preceding,  is  yet 
without  the  slightest  support  from  the  ontogeny  of 
the  group  under  review. 

"It  is  a  matter  of  easy  demonstration  that  in  many 
species  of  hydroids  the  egg  may  be  followed  in  every 
detail  from  its  origin  as  an  ectoderm  or  an  entoderm 
or  interstitial  cell  through  its  gradual  differentiation 
and  growth  to  maturation,  as  a  distinct  individual 
cell,  without  the  slightest  tendency  to  multiplication." 


96  GERM-CELL   CYCLE   IN  ANIMALS 

"It  is  passing  strange  that  he  should  ignore  the 
body  of  facts  concerned  in  regeneration,  and  among 
them  the  reproductive  organs.  And  it  is  still  more 
strange  that  in  support  of  this  he  should  cite  in 
detail  the  Hydrozoa  as  illustrating  and  supporting 
the  hypothesis,  ignoring  the  well-known  facts  that 
among  these  are  abounding  evidences  which  afford 
insuperable  objections  to  just  these  assumptions. 
The  present  author  has,  in  many  cases,  shown  that 
gonads  may  be  as  readily  regenerated  by  hydroids 
and  medusae  as  any  other  organs ;  and  that  not  for 
once  or  twice,  but  repeatedly  in  the  same  specimen, 
and  that  de  novo  and  in  situ;  not  the  slightest  evi- 
dence being  distinguishable  that  any  migration 
through  preexisting  *  germ-tracks'  occurred.  The 
assumption  that  in  these  animals  the  gonads  have 
*  been  shifted  backwards  in  the  course  of  phylogenetic 
evolution,  that  is,  have  been  moved  nearer  to  the 
starting  point  of  development'  seems  so  at  variance 
with  known  facts  as  to  be  difficult  to  appreciate  or 
respect." 

Professor  Hargitt  finally  concludes  with  the  fol- 
lowing sentence :  "I  believe  the  foregoing  facts 
must  suffice  to  show  that,  both  as  to  origin,  differen- 
tiation, and  growth,  the  germ-cells  of  the  Hydrozoa, 
so  far  from  sustaining  the  doctrine  of  the  germ- 
plasm,  afford  the  strongest  and  most  direct  evidence 
to  the  contrary." 

G.  T.  Hargitt  (1913)  has  also  discovered  facts 
regarding  the  history  of  the  germ  cells  in  coelenter- 
ates   which  are  decidedly  opposed  to  Weismann's 


PORIFERA,   CCELENTERATA,   VERTEBRATA     97 

views.  He  finds  that  "The  egg  cells  of  Campanularia 
flexuosa  arise  in  the  entoderm  of  the  pedicel  of  the 
gonophore,  by  the  transformation  of  a  single 
epithelial  cell,  or  from  the  basal  half  of  a  divided 
cell,  the  distal  half  of  which  remains  an  epithelial 
cell  and  retains  its  epithelial  functions.  Therefore 
the  egg  cells  have  come  from  differentiated  body- 
cells  (so-called)  and  there  is  no  differentiation  of 
the  germ-plasm  in  the  sense  that  germ-cells  are 
early  differentiated  and  set  aside  and  do  not  partici- 
pate in  the  body  functions.  Any  cell  of  the  ento- 
derm of  Campanularia  flexuosa  may  become  an  egg 
cell  if  it  is  in  the  position  of  the  developing  gono- 
phore "  (p.  411). 

In  spite  of  these  attacks  upon  the  germ-plasm 
theory  as  applied  to  ccelenterates,  the  possibility 
and  even  probability  of  such  a  condition  seems  to 
the  writer  to  exist,  and  he  is  inclined  to  accept 
Downing's  position  in  the  matter.  Weismann's 
views  must,  however,  be  modified,  since  the  germ 
cells  are  not  ectoderm  cells,  as  he  claims,  nor  do 
they  belong  to  any  germ  layer.  They  are,  according 
to  the  view  adopted  here,  set  aside  as  a  separate 
class  of  cells  at  some  stage  during  early  development, 
are  scattered  about  among  the  cells  of  the  ectoderm 
or  entoderm,  depending  upon  the  species,  or  lie  in 
the  mesoglea.  We  know  that  external  conditions 
may  stimulate  reproductive  activity  in  certain 
ccelenterates  (Frischholz,  1909)  and  consequently 
the  development  of  germ  cells,  and  we  must  conclude 
that  these  germ  cells  are  present  at  all  times  in  a 


98  GERM-CELL  CYCLE  IN  ANIMALS 

more  or  less  dormant  condition,  just  as  they  are  in 
more  complex  animals.  Furthermore,  the  germ  cells 
must  be  widely  scattered,  as  has  been  shown  by  Harm 
(1902)  in  the  young  hydranths  of  Clava,  by  Wulfert 
(1902)  in  the  planula  of  Gonothyrcea,  and  by  Small- 
wood  (1909)  in  the  polyp  of  Hydractinia.  This  wide 
distribution  of  primitive  germ  cells  accounts  for  the 
reproductive  powers  of  regenerated  pieces  of  hy- 
droids. 

3.    Vertebrata 

Efforts  have  been  made  by  many  investigators  to 
trace  the  keimbahn  in  vertebrates,  but  thus  far  no 
method  has  yet  been  devised  which  will  enable  us 
to  distinguish  germ  cells  from  other  cells  in  the  early 
embryonic  stages.  That  we  shall  be  able  to  recognize 
germ  cells  in  still  earlier  stages  of  development  than 
has  yet  been  accomplished  seems  certain,  and  the 
recent  contributions  of  Rubaschkin  (1910),  Tschasch- 
kin  (1910),  von  Berenberg-Gossler  (1912a)  and 
Swift  (1914)  have  already  made  considerable  ad- 
vances by  the  use  of  some  of  the  more  modern  cyto- 
logical  methods.  Three  principal  theories  have  been 
advanced  regarding  the  origin  of  the  germ  cells  in 
vertebrates,  and  these  will  be  briefly  stated  before 
the  histories  of  the  germ  cells  in  special  cases  are 
discussed. 

The  germinal  epithelium  theory  was  advanced  by 
Waldeyer  in  1870.  At  that  time  nothing  was  known 
regarding  the  migration  of  germ  cells  during  the 
embryonic    development    of    vertebrates,    and   it  is 


PORIFERA,   CCELENTERATA,   VERTEBRATA     99 

not  strange  that  he  should  have  come  to  the  con- 
clusion that  the  primordial  ova  arise  from  the 
epithelial  cells  of  the  genital  ridge  among  which  they 
were  observed.  Although  this  theory  was  accepted 
by  most  embryologists,  it  has  gradually  been  aban- 
doned until  now  it  has  very  few  supporters. 

The  gonotome  theory  resulted  from  the  studies  of 
Ruckert  (1888)  and  Van  Wijhe  (1889).  The  germ 
cells  appeared  to  these  investigators  to  arise  in  a 
part  of  the  segmental  mesoblast  of  the  embryo  to 
which  the  latter  applied  the  term  'gonotome.' 
From  the  gonotome  they  become  embedded  in  the 
peritoneum.  Thus  the  same  cells  are  recognized  as 
germ  cells  by  the  adherents  of  both  theories,  but  a 
difference  exists  regarding  their  origin. 

The  theory  of  early  segregation  has  become  the 
most  prevalent  view  of  the  origin  of  the  germ  cells 
of  vertebrates,  although  there  are  many  who  still 
hold  one  of  the  other  hypotheses.  According  to 
this  theory  the  germ  cells  are  set  aside  during  the 
early  embryonic  stages  before  definite  germ  layers 
are  formed,  and  they  later  arrive  at  the  germinal 
ridge  either  by  their  own  migration  or  by  changes  in 
the  position  of  the  tissues  during  development. 
The  germinal  epithelium  theories  have  little  if  any 
evidence  in  their  favor,  since  no  one  has  actually  ob- 
served a  transformation  of  peritoneal  or  mesoblast 
cells  into  germ  cells.  On  the  other  hand,  there  is  an 
abundance  of  proof  that  these  cells  migrate  from 
some  distance  into  the  position  of  the  sex  glands. 

According  to  Dustin   (1907),   Firket   (1914)  and 


100        GERM-CELL   CYCLE   IN  ANIMALS 

several  others  there  are  two  methods  of  origin,  and 
primary  and  secondary  sex  cells  are  produced.  The 
former  are  probably  derived  from  the  blastomeres; 
whereas  the  secondary  sex  cells  are  entirely  inde- 
pendent and  arise  from  the  coelomic  epithelium. 

The  first  statement  of  the  theory  of  early  segre- 
gation was  made  by  Nussbaum  (1880),  who  studied 
the  history  of  the  germ  cells  in  the  trout.  Following 
Nussbaum,  Eigenmann  (1892,  1896)  contributed  to 
the  support  of  the  theory  by  his  investigations  on 
the  viviparous  teleost,  Cymatogasier.  This  proved 
to  be  excellent  material  for  such  studies  and  led 
Eigenmann  to  the  conclusion  that  the  germ  cells 
are  set  aside  in  this  fish  during  the  early  cleavage 
stages  of  the  egg,  probably  at  the  thirty-two  cell 
stage.  In  other  cases  it  has  been  impossible  to 
trace  the  germ  cells  back  to  such  an  early  embryonic 
condition,  but  nevertheless  the  evidence  has  been 
almost  uniformly  in  favor  of  early  segregation. 
Some  of  those  who  have  advocated  such  an  early 
origin  of  germ  cells  are  Wheeler  (1900)  in  the  lamprey, 
Beard  (1900,  1902)  in  Raja  and  Pristiurus,  Nussbaum 
(1901)  in  the  chick.  Woods  (1902)  in  Acanthais, 
Allen  (1906,  1907,  1909,  1911)  in  Chrysemys,  Rana, 
Amia,  and  Lepidosteus,  Rubaschkin  (1907,  1909, 
1910,  1912)  in  the  chick,  cat,  rabbit,  and  guinea-pig, 
Kuschakewitsch  (1908)  in  Ra7ia,  Jarvis  (1908) 
in  Phrynosoma,  Tschaschkin  (1910)  in  the  chick, 
von  Berenberg-Gossler  (1912)  in  the  chick,  Schapitz 
(1912)  in  Amhlystoma,  Fuss  (1912)  in  the  pig  and 
man,  and  Swift  (1914)  in  the  chick.     This  is  by  no 


PORIFERA,   COELENTERATA,   VERTEBRATA    101 

means  a  complete  list  but  indicates  the  range  of 
forms  studied  and  the  current  interest  in  this  subject. 

Some  of  the  characteristics  by  means  of  which 
germ  cells  can  be  distinguished  in  vertebrate  embryos 
are  as  follows :  (1)  the  presence  of  yolk,  (2)  an 
ameboid  shape,  (3)  large  size,  and  (4)  slight  staining 
capacity.  By  sectioning  embryos  of  various  ages 
the  changes  in  position  of  the  germ  cells  can  be  fol- 
lowed with  considerable  accuracy.  Most  investi- 
gators agree  that  the  movement  of  the  germ  cells  from 
the  tissues  where  first  observed  to  the  genital  ridge 
is  caused  by  ameboid  activities  of  the  cells  themselves 
and  by  changes  in  the  position  of  the  organs  of  the 
embryo.  The  paths  of  migration  of  four  verte- 
brates, a  turtle,  Chrysemys,  sl  frog,  Rana,  the  gar 
pike,  Lepidosteus,  and  the  fresh- water  dogfish,  Amia, 
are  shown  in  Fig.  6.     For  example  : 

*'In  Lepidosteus  the  sex-cells  [Fig.  6,  3,  SI]  first 
seen  in  the  ventral  and  lateral  portions  of  the  gut- 
entoderm  [Int]  migrate  to  occupy  a  position  in  the 
dorsal  portion  of  it,  from  which  they  pass  dorsally 
into  the  loose  mesenchyme  that  forms  the  substance 
of  the  developing  mesentery  [l/e^].  As  the  mesen- 
tery becomes  more  narrow  and  compact,  owing  to 
the  increase  in  size  of  the  body  cavity,  the  sex  cells 
migrate  to  its  dorsal  portion  and  laterally  to 
the  sex-gland  anlagen  (Fig.  6,  4»  Sc).  Roughly 
speaking,  one-half  of  the  total  number  of  sex-cells 
reach  the  sex-gland  anlagen,  the  remainder  being 
distributed  between  the  intestinal  entoderm,  the 
mesodermal  layers  of  the  intestine,  the  mesentery, 


102        GERM-CELL   CYCLE   IN  ANIMALS 

and  the  tissues  at  and  dorsal  to  the  root  of  the 
intestine"  (Allen,  1911,  p.  32). 

Of  the  more  recent  investigations,  facts  discov- 
ered by  Dodds  (1910),  Rubaschkin  (1910,  1912), 
Tschaschkin  (1910),  von  Berenberg-Gossler  (1912), 
and  Swift  (1914)  are  especially  worthy  of  mention. 
Dodds  (1910)  found  that  in  the  teleost,  Lophius, 
the  germ  cells  in  the  embryos  cannot  be  definitely 
distinguished  previous  to  the  appearance  in  their 
cytoplasm  of  a  body  which  stains  like  a  plasmosome 
(Fig.  31,  A).  Germ  cells  are  undoubtedly  segregated 
before  this  period,  but  they  exhibited  no  characteris- 
tics with  the  methods  employed  which  rendered  them 
distinguishable.  Dodds  believes  that  this  cyto- 
plasmic body  is  extruded  plasmosome  material, 
probably  part  of  one  of  the  two  plasmosomes  pos- 
sessed by  many  of  the  cells  at  this  period. 

Rubaschkin,  in  1910,  announced  the  results  ob- 
tained with  the  eggs  of  the  guinea-pig  by  certain 
methods  designed  to  bring  into  view  the  chondrio- 
somes.  He  shows  that  the  chondriosomes  of  the 
undifferentiated  cells  are  granular,  and  that  as 
differentiation  proceeds,  these  granules  unite  to 
form  chains  and  threads  (Fig.  31,  B).  The  sex 
cells,  however,  retain  the  chondriosomes  in  their 
primitive  granular  form,  and  remain  in  an  undiffer- 
entiated condition  situated  in  the  posterior  part  of 
the  embryo  among  the  entoderm  cells.  Tschaschkin 
(1910),  in  the  same  year,  came  to  a  similar  conclusion 
from  studies  made  with  chick  embryos.  Rubaschkin 
(1912)  has  also  extended  his  investigations  on  guinea- 


PORIFERA,   COELENTERATA,   VERTEBRATA    103 


pig  embryos.  The  accompanying  diagram  (Fig.  3^2) 
shows  the  fertilized  egg  and  the  early  cleavage  cells 
all  alike  (in  black) ;  some  of  their  descendants  become 
differentiated   into   the  somatic   cells   of   the  germ 


Fig.  31.  —  Germ  cells  of  vertebrates.  A.  From  embryo  of  the  teleost, 
Lophius,  with  plasmosome  (?)  extruded  into  cytoplasm.  (From 
Dodds,  1910.)  B.  One  germ  cell  and  four  somatic  cells  from  a 
guinea-pig  embryo.  {From  Rubaschkin,  1912.)  C.  Germ  cell  of 
chick  showing  "  Netzapparat."  {From  von  Berenberg-Gossler,  1912.) 
D.  Primordial  germ  cell  {g)  and  blood  cell  (6)  in  lumen  of  blood 
vessel  {D  of  a  nineteen  somite  chick  embryo,  a  =  attraction-sphere. 
(From  Swift,  1914.) 

layers  (circles),  but  others  (in  black)  remain  in  a 
primitive  condition  and  are  recognizable  as  the 
primordial  germ  cells  ;  these  remain  at  rest  for  a 
considerable  period,  but  finally  multiply  and  become 
part  of  the  germinal  epithelium  (g.ep). 


104        GERM-CELL   CYCLE   IN  ANIMALS 

Von  Berenberg-Gossler  (1912)  considers  the  "Netz- 
apparat"  in  the  primitive  germ  cells  of  the  chick  of 
particular  importance  (Fig.  31,  C),  comparing  it 
with  the  "wurstformige  Korper  "  described  by  Hasper 


>d< 


A 


;^ 


FiG.  32.  —  Diagram  to  show  the  history  of  the  germ  cells  in  the  embryo 
of  the  guinea-pig.  g.ep  =  germinal  epithelium.  (From  Rubasch- 
kin,  1912.) 


(1911)  in  Chironomus  (p.  108,  Fig.  33).  The  ap- 
pearance of  this  structure  in  *'Keimbahnzellen" 
is  thought  to  be  due  to  the  long  period  during  which 
these  cells  do  not  divide.  Duesberg  (191*2),  however, 
after  an  exhaustive  review  of  the  literature  on  this 


PORIFERA,   CCELENTERATA,   VERTEBRATA    105 

structure  concludes  that  it  is  not  a  special  cell  organ 
but  an  artifact.  Kulesch  (1914),  on  the  contrary, 
finds  it  to  be  a  constant  organ  in  the  eggs  of  the  cat, 
dog,  and  guinea-pig. 

The  evidence  of  a  continuous  germ-cell  cycle  in 
the  vertebrates  is  more  convincing  than  in  the 
sponges  and  coelent crates,  and  leads  us  to  predict 
that  it  will  not  be  long  before  the  gap  still  existing 
during  which  germ  cells  cannot  be  recognized  will 
be  filled  in  to  the  satisfaction  of  the  majority  of 
investigators. 


CHAPTER  V 

THE     SEGREGATION     OF    THE     GERM     CELLS     IN 

THE  ARTHROPODA 

1.  The  Keimbahn  in  the  Insects 

The  insects  have  furnished  a  very  large  proportion 
of  the  data  upon  which  many  of  our  biological 
conceptions  are  now  based,  and  they  are  becoming 
more  and  more  popular  for  studies  of  the  physical 
basis  of  heredity,  and  for  purposes  of  animal  breeding. 
It  was  in  insects  (Miastor)  that  the  early  segrega- 
tion of  the  germ  cells  in  animals  was  first  definitely 
established.  The  accessory  chromosome  was  dis- 
covered in  insects  by  Henking  in  1891,  and  our 
knowledge  of  the  chromosomes,  which  has  increased 
so  remarkably  within  the  past  fifteen  years,  is  due 
principally  to  the  study  of  oogenesis  and  spermato- 
genesis in  insects.  In  this  chapter  the  chromosomes 
will  only  be  considered  incidentally,  a  more  detailed 
account  being  deferred  until  later  (Chapter  IX). 
The  early  history  of  the  germ  cells  in  insect  develop- 
ment has  not  been  slighted,  for  there  are  many 
reports  based  on  this  subject  alone  and  still  more 
data  hidden  away  in  contributions  on  general  em- 
bryology. It  will  be  necessary  here  to  select  from 
this  abundance  of  material  those  reports  that  give 
us   the   clearest   pictures    of   the   keimbahnen.     As 

106 


GERM   CELLS   IN  THE  ARTHROPODA     107 

usual,  certain  species  or  groups  of  species  have 
proven  more  favorable  than  others  for  germ-cell 
studies,  especially  those  belonging  to  the  orders 
DiPTERA,  CoLEOPTERA,  and  Hymenoptera. 

DiPTERA.  Robin,  in  1862,  described  what  he 
called  "globules  polaries"  at  one  end  of  the  nearly 
transparent  eggs  of  the  crane  fly,  Tipulides  culici- 
formes,  and  the  following  year  Weismann  (1863)  re- 
ported the  formation  of  similar  cells,  the  "Pol- 
zellen"  at  the  posterior  end  of  the  eggs  of  the  midge, 
Chironomus  nigroviridis,  and  the  blow  fly,  Calliphora 
(Musca)  vomitoria.  It  remained  for  Leuckart  (1865) 
and  Metchnikoff  (1865,  1866),  however,  to  identify 
the  pole  cells  (in  Miastor)  as  primordial  germ  cells ; 
their  results  were  confirmed  for  Chironomus  by 
Grimm  (1870)  and  Balbiani  (1882,  1885). 

Pole  cells  have  also  been  described  among  the 
DiPTERA,  in  Musca  by  Kowalevsky  (1886),  Voeltz- 
kow  (1889),  and  Escherich  (1900);  in  Calliphora 
by  Graber  (1889)  and  Noack  (1901) ;  in  Chironomus 
by  Ritter  (1890)  and  Hasper  (1911) ;  in  Lucilia  by 
Escherich  (1900) ;  in  Miastor  by  Kahle  (1908)  and 
Hegner  (1912,  1914a),  and  in  Compsilura  by  Hegner 
(1914a). 

Four  genera  of  flies  will  serve  to  illustrate  the 
methods  of  germ-cell  segregation  in  this  order :  (1) 
Chironomus  (Ritter,  1890;  Hasper,  1911),  (2)  Cal- 
liphora (Noack,  1901),  (3)  Miastor  (Kahle,  1908; 
Hegner,  1912,  1914a),  and  (4)  Compsilura  (Hegner, 
1914a).  Since  Miastor  has  been  discussed  in  detail 
in  Chapter  III  it  will  be  only  briefly  referred  to  here. 


108        GERM-CELL   CYCLE   IN  ANIMALS 

We  owe  the  first  accurate  account  of  the  germ 
cells  in  Chironomus  to  Ritter  (1890),  who,  by  means 
of  the  section  method,  showed  that  the  "y^lk 
granules"  described  by  Weismann  (1863)  in  the 
pole  cells  are  derived  from  a  disc-shaped  mass  of 
substance  situated  near  the  posterior  end  of  the  egg 
and  termed  by  him  the  "Keimwulst."  Hasper 
(1911)  was  able  to  confirm  this  discovery,  to  add 
other  interesting  facts,  and  to  correct  several  of 
Ritter 's  errors.  The  "Keimwulst"  of  Ritter  is 
called  by  Hasper  the  "Keimbahnplasma." 

Ritter  advanced  the  idea  that  the  cleavage 
nucleus  of  Chironomus  divides  within  the  "Keim- 
wulst"  and  that  here  the  first  cleavage  division 
occurs,  one  daughter  nucleus  remaining  in  the  "Keim- 
wulst"  and  becoming  the  center  of  the  primordial 
germ  cell,  the  other  giving  rise  to  somatic  nuclei. 
This  is  probably  the  basis  for  Weismann's  (1904) 
statement  regarding  his  conception  of  the  germ- 
plasm  that,  "If  we  could  assume  that  the  ovum, 
just  beginning  to  develop,  divides  at  its  first  cleavage 
into  two  cells,  one  of  which  gives  rise  to  the  whole 
body  (soma)  and  the  other  only  to  the  germ-cells 
lying  in  this  body,  the  matter  would  be  theoretically 
simple.  ...  As  yet,  however,  only  one  group  of 
animals  is  known  to  behave  demonstrably  in  this 
manner,  the  Diptera  among  insects.  .  .  ."  There 
is,  however,  nothing  in  the  literature  to  warrant 
the  above  statement,  since  Ritter's  hypothesis 
has  been  disproved  by  Hasper. 

According  to  Hasper  one  of  the  cleavage  nuclei 


GERM   CELLS   IN   THE   ARTHROPODA     109 

at  the  four  cell  stage  becomes  separated  from  the 
rest  of  the  egg,  together  with  all  of  the  Keimbahn- 
plasma  as  the  primordial  germ  cell  (Fig.  33  B, 
p.g.c).  The  Keimbahnplasma  is  apparently  equally 
divided  between  the  daughter  cells  when  the 
primordial  germ  cell  divides.  Later  the  nuclei 
of  the  germ  cells  increase  in  number  without  an 
accompanying  division  of  the  cell,  thus  producing 
binucleated  cells  (Fig.  33,  C).  The  history  of  the 
pole  cells  during  embryonic  development  will  be 
more  fully  described  in  the  Coleoptera,  since  in 
the  beetles  the  Keimbahn  is  much  more  distinct. 
The  origin  and  nature  of  the  Keimbahnplasma 
was  not  determined  by  Hasper,  but  it  was  found  to 
persist  in  certain  cases  even  until  the  larval  stage 
was  reached  (Fig.  33,  D). 

In  Calliphora  Noack  (1901)  described  a  dark 
granular  disc  at  the  posterior  end  of  the  egg  (Fig.  34) 
which  he  termed  the  "  Dotterplatte "  and  which, 
like  the  pole-plasm  of  Miastor  and  the  Keimbahn- 
plasma of  Chironomus  takes  part  in  the  formation  of 
the  primordial  germ  cells.  The  eggs  of  the  parasitic 
fly,  Compsilura  concinnata,  were  also  found  by  the 
writer  (Hegner,  1914a)  to  possess  a  granular  pole- 
disc,  thus  adding  one  more  species  to  the  list  of 
DiPTERA  in  which  such  a  structure  exists. 

Coleoptera.  The  origin  of  the  germ  cells  in 
beetles  and  their  subsequent  history  are  well  known 
only  in  certain  species  of  the  family  Chrysomelid^ 
of  the  genera  Calligrapha  and  Leptinoiarsa.  The 
contributions  of  Wheeler   (1889),  Lecaillon   (1898), 


Q: 


'BO,:^  OJO;^ 


ng.c. 


Fig.  33.  —  Chironomus.  A.  Longitudinal  section  through  the  posterior 
end  of  a  freshly  laid  egg.  B.  Longitudinal  section  through  egg 
during  division  of  first  four  cleavage  nuclei ;  at  posterior  end  the 
primordial  germ  cell  is  just  being  formed.  C.  One  of  primordial 
germ  cells  containing  two  nuclei  and  remains  of  "  Keimbahnplasma." 
D.  Germ  gland  of  the  larva  in  which  remains  of  "  Keimbahnplasma  " 
still  appear.  Khpl  =  "  Keimbahnplasma  "  ;  p.g.c.  =  primordial  germ 
cell.      {Froyn  Hasper,  1911.)  (HO) 


GERM   CELLS   IN   THE   AUTHROPODA     111 


Hegner  (1908,  1909a,  19096,  1911a,  19116,  1914a), 
and  Wieman  (1910a,  19106)  will  be  referred  to  in 
the  following  paragraphs. 

Wheeler  (1889)  figured  several  primordial  germ 
cells  in  an  egg  of  Leptinotarsa  with  a  segmented  germ 
band  and  suspected 
their  true  nature,  but 
did  not  discover  them 
in  earlier  stages.  Le- 
caillon  (1898)  de- 
scribed the  pole-cells 
in  several  chrysomelid 
beetles,  but  did  not 
make  out  any  of  the 
details  concerning 
their  origin,  structure, 
and  migrations. 

Within  the  last 
seven  years  the  writer 
has  devoted  a  consid- 
erable portion  of  his 
time  to  morphological 
and  experimental 
studies  of  the  eggs  of 
beetles,  particularly 
Calligrapha  higshjana,  C.  multipunctata,  C.  lunata, 
and  Leptinotarsa  decemlineata.  The  eggs  of  these 
species  are  peculiarly  favorable  for  study,  since  they 
are  definitely  oriented  in  the  body  of  the  mother 
and  various  surfaces  can  be  recognized  in  the  newly 
laid  egg :   they  can  be  placed  under  the  most  severe 


Fig.  3i.  —  Calliphora.  A.  Longitudi- 
nal section  through  posterior  end  of 
freshly  laid  egg,  showing  "  Dotter- 
platte  (Dpi).  B.  Longitudinal  sec- 
tion through  posterior  end  of  egg  at 
time  of  blastoderm  formation,  showing 
protrusion  of  primordial  germ  cells 
(p.ff.c).     (From  Noack,  1901.) 


112        GERM-CELL   CYCLE  IN  ANIMALS 

experimental  conditions  without  killing  them  or 
stopping  their  progressive  development ;  and  they 
can  be  killed,  fixed,  sectioned,  and  stained  with 
comparative  ease.  Furthermore,  the  eggs  of  these 
beetles  possess  a  well-defined  pole-disc,  and  the 
primordial  germ  cells  which  arise  even  before  the 
blastoderm  is  formed  are  easily  distinguishable 
from  the  somatic  cells  and  thus  can  be  traced  from 
the  time  of  their  appearance  until  they  become  ma- 
ture eggs  and  spermatozoa. 

The  ova  of  insects  have  long  been  considered 
among  the  most  highly  organized  of  all  animal 
eggs.  That  they  are  definitely  oriented  while  still 
within  the  ovary  was  expressed  by  Hallez  (1886)  in 
his  "Law  of  the  Orientation  of  Insect  Embryos" 
as  follows :  "The  cell  possesses  the  same  orientation 
as  the  maternal  organism  that  produces  it ;  it  has  a 
cephalic  pole  and  a  caudal  pole,  a  right  side  and  a 
left  side,  a  dorsal  surface  and  a  ventral  surface; 
and  these  different  surfaces  of  the  egg-cell  coincide 
to  the  corresponding  surfaces  of  the  embryo."  The 
orientation  of  an  ovarian  egg  is  indicated  in  Fig.  35, 
and  here  also  is  shown  the  position  and  surfaces  of 
the  egg  at  the  time  of  deposition.  When  the  egg  is 
laid  the  beetle  clings  to  the  under  surface  of  a  leaf, 
and  with  a  drop  of  viscid  substance  from  the  acces- 
sory glands  of  the  reproductive  organs,  fastens  the 
egg  by  its  posterior  end  (p)  to  the  leaf ;  then  with  the 
tip  of  the  abdomen  the  egg  is  pushed  back  through 
the  arc  indicated  by  the  dotted  line.  It  is  a  simple 
matter  to  determine  the  various  surfaces  of  eggs 


GERM   CELLS   IN   THE   ARTHROPOD  A     113 

laid  in  this  manner.  Gravity  apparently  has  no 
influence  upon  the  development,  since  eggs  in  a  state 
of  nature  occupy  all  positions  with  respect  to 
this  factor  without  becoming  altered  in  any  way. 
Only  one  case  has  come  to  the  writer's  attention 
of  an  influence  of  gravity  in  insect  development  — 
the  eggs  of  the  water  beetle,  Hydrophiliu-  atterimus. 


,  p    r      d     X     a 


Fig.  35.  —  A  diagramatic  drawing  of  Calligrapha  bigsbyana  clinging  to 
the  under  side  of  a  willow  leaf  and  showing  the  orientation  of  the 
egg  in  the  ovarian  tubule  and  after  deposition,  a  =  anterior ;  d  = 
dorsal  ;  p  =  posterior  ;  r  —  right  side  ;  x  =  place  where  egg  was 
marked  with  India  ink  as  means  of  orientation  after  removal  from 
leaf. 


according  to  Megusar  (1906),  develop  abnormally  if 
the  cocoon  in  which  they  are  laid  is  inverted. 

The  events  that  precede  the  establishment  of  the 
primordial  germ  cells  in  chrysomelid  beetles  may  be 
described  briefly  as  follows :  The  egg,  when  laid 
(Fig.  36,  A),  consists  of  a  large  central  mass  of  yolk 
globules  (y),  among  which  are  very  fine  strands  of 
cytoplasm  ;  a  thin  peripheral  layer  of  cytoplasm,  the 
''keimhautblastem"  of  Weismann  (JxJihl),  a  delicate 
vitelline  membrane  (v.m.),  a  chitinous  shell,  the 
chorion,  and  a  nucleus  consisting  of  the  egg  nucleus 


•ga 


...khbl 


-:.■.... .y. 


-••v.m. 


B 


-...fH 


'•:.'^-''.."-'v.-- •■••.'•  •■'.•■■•■'.•■.■  ^..''v'"-"V.-*Vv:!.?v.'V"v).";;;>' ■:..";•••.■'•.  •..•.■-■  •;.l :  '•"..•  V]-*  •;•;.■".•.•■ ':  '•.  .■*■:: 
••:.•::..•  •.■:■..".■..•  .•.■;•;■■■;..■ .  ■*  .  ■.-■.•..  >.•:..•■•  .•"■•'•■•vvv/.;|"::;  .• : . ;  •.•.•..•■■.•..•"...  •■•  •■  •.■••.'•;".■.••-"••= 


Fig.  3,.-Calli,rapha.     A.  Long^udinal  section  ^^^-^^  ^J^^^^^^ 
biasbyana   four   hours    after    deposition.     B.  Longitudinal    sec 
through  an  egg  of  C.  biosbyana  14  hours  after  deposition      C.Jjo 
germ  cells  just  protruding  from  posterior  end  of  egg  of  C.rnulH 
fuZata      D    The  pole-disc  in  an  egg  of  C.  mulhpunctata     g.c.d  - 
pole  dtc-  gn.  =  germ  nuclei  fusing;  khbl  =  keimhautblastem  ;  P   - 
posterior 'end  of  egg;  pM.n.  =  preblastodermic  nuclei;  v.m.  =  vrtel- 
Une  membrane  ;  vt.  =  vitellophags  ;  y.  =  yolk.  ^ 


GERM   CELLS   IN   THE   ARTIIROPODA     115 

and  a  sperm  nucleus  combined  (g.n).  Frequently 
the  two  polar  bodies  have  not  yet  been  produced 
when  the  egg  is  laid  and  thus  many  stages  may  be 
encountered  in  the  newly  laid  eggs.  Polyspermy  is 
a  normal  condition  in  insects  and  several  sperma- 
tozoa are  often  observed  among  the  yolk  globules. 
The  keimhautblastem  is  not  homogeneous  through- 
out, for  at  the  posterior  end  there  is  embedded  in  it  a 
disc-shaped  mass  of  darkly  staining  granules  which  I 
have  called  the  pole-disc  (g.c.d.)  and  which  resembles 
the  pole-plasm  of  Miastor,  the  "Keimwulst"  or 
"  Keimbahnplasma  "  of  Chironomus  and  the  "  Dotter- 
platte"  of  Calliphora, 

The  cleavage  nucleus  divides  by  mitosis;  the 
daughter  nuclei  separate  slightly,  and  divide;  and 
this  process  is  continued  until  nuclei,  each  surrounded 
by  a  small  mass  of  cytoplasm,  are  scattered  more  or 
less  regularly  throughout  the  egg.  Then  a  division 
of  the  nuclei  into  tw^o  groups  occurs;  those  of  one 
group  migrate  to  the  periphery,  fuse  with  the  periph- 
eral layer  of  cytoplasm,  and  are  cut  off  by  cell  walls, 
thus  forming  the  blastoderm;  whereas  the  other 
nuclei,  the  vitellophags,  remain  behind  among  the 
yolk  globules  which  it  is  their  function  to  dissolve. 
The  blastoderm  consists  of  a  single  layer  of  cells, 
except  at  the  posterior  end  where  its  formation  has 
been  interrupted  by  the  process  resulting  in  the 
establishment  of  the  primordial  germ  cells. 

The  primordial  germ  cells  are  formed  in  the  fol- 
lowing manner.  The  cleavage  nuclei  at  the  posterior 
end  of  the  egg  that  encounter  the  pole-disc  granules 


116        GERM-CELL   CYCLE   IN  ANIMALS 

behave  differently  from  those  at  other  points,  since 
they  do  not  remain  to  form  part  of  the  blastoderm 
but  continue  to  migrate  until  they  have  become 
entirely  separated  from  the  rest  of  the  egg.  During 
this  process  each  of  the  sixteen  nuclei  that  act  in  this 
way  becomes  surrounded  by  a  halo  of  granules  — 
part  of  the  pole-disc.  Then  cell  walls  appear  and 
sixteen  primordial  germ  cells  result.  These  form  a 
group  at  the  posterior  end,  each  member  of  which 
divides  twice,  thus  producing  sixty-four  germ  cells 
in  all.  During  these  divisions,  which  are  mitotic, 
the  pole-disc  granules  appear  to  be  equally  distrib- 
uted between  the  daughter  cells  (Fig.  37,  B). 
A  rest  period  then  occurs,  as  far  as  cellular  multipli- 
cation is  concerned,  during  which  a  ventral  plate, 
which  later  grows  into  the  germ  band,  develops  on 
the  ventral  surface  of  the  egg.  As  in  Miastor  the 
germ-band  pushes  around  on  the  dorsal  surface 
and  the  group  of  sixty-four  germ  cells  is  carried 
along  with  it.  In  the  meantime  the  germ  cells 
begin  to  migrate  from  the  amniotic  cavity  in  which 
they  lie  through  a  sort  of  canal  at  the  bottom  of  a 
groove  in  the  germ-band  and  thus  make  their  way 
inside  of  the  embryo  (Fig.  37,  F).  That  the  germ 
cells  actually  migrate  and  are  not  simply  forced 
about  by  the  surrounding  tissues  seems  certain  since 
they  are  ameboid  in  shape  and  pseudopodia  extend 
out  in  the  direction  of  their  movement  (Fig.  37,  F) . 

After  penetrating  into  the  embryo  the  germ  cells 
become  separated  into  two  groups.  It  was  difficult 
to   count   the   number   in   each   group,    but   many 


GERM   CELLS   IN   THE   ARTHROPOD  A     117 


•Pdg. 


V.2.-.-\ 


-.'■• 

#^--khbI 

- 0 

y. 

-0: 

^.~^^... 

SP^ 

-  — go.d. 

G 



•---n 

PgC 


&a. 


•R 


I 


Fig.  37.  —  Calligrapha.  A.  A  germ  cell  of  C.  maltijnmctala  shortly  after 
being  cut  off  from  the  egg.  B.  Division  of  a  primordial  germ  cell. 
C.  Longitudinal  section  through  egg  of  C.  biysbyana  at  blastoderm 
stage ;  the  posterior  end  was  killed  with  a  hot  needle  just  after 
deposition.  D.  Longitudinal  section  through  uninjured  egg  at 
same  stage.  E.  Two  ectoderm  cells  (e),  two  mesoderm  cells  (m), 
and  two  germ  cells  (g.c.)  from  an  egg  three  days  old.  F.  Germ 
cell  during  migration  into  the  embryo  (three  days  old).  G.II.I.  Longi- 
■  tudinal  sections  through  eggs  centrifuged  for  one  hour,  two  hours, 
and  four  hours  respectively,  bl  =  blastoderm ;  g.c.d.  =  granules  of 
pole-disc  ;  A:  =  killed  portion  of  egg  ;  khhl.  =  keinihaut  hi  astern ;  p.  = 
posterior;  7>f;r  =  primordial  germ  cells;  v  —  vitcllophags ;  v.z.  = 
vesicular  zone  ;  j.  ~  yolk. 


118        GERM-CELL  CYCLE  IN  ANIMALS 

attempts  seem  to  justify  the  conclusion  that  the 
division  is  equal  or  approximately  equal,  that  is, 
each  group  contains  about  thirty-two  germ  cells. 
These  groups  acquire  a  covering  of  mesoderm  cells, 
are  carried  by  the  somatic  tissues  to  a  position 
near  the  dorsal  surface  on  either  side  of  the  body  in 
the  last  two  abdominal  segments,  and  thus  become 
germ  glands  situated  in  their  definite  positions. 
Some  time  before  the  larval  stage  is  reached,  the 
sex  of  the  embryo  can  be  determined  by  the  shape 
of  the  germ  glands ;  those  of  the  male  become  dumb- 
bell shape,  whereas  the  female  organs  retain  the  earlier 
pear  shape  and  begin  to  acquire  terminal  filaments. 
It  is  interesting  to  note  that  much  time  and 
effort  have  been  wasted  by  those  who  have  attempted 
to  influence  the  sex  of  caterpillars  by  over-feeding 
or  starving.  Kellogg  (1907),  for  example,  "dis- 
covered," after  an  unsuccessful  attempt  to  change  the 
sex  of  silk  worms  by  this  means,  that  these  cater- 
pillars already  possess  germ  glands  which  are  dif- 
ferentiated as  male  or  female.  If  he,  and  others 
who  have  undertaken  similar  experiments,  had 
examined  the  literature  on  the  origin  of  the  germ 
cells  in  insects,  they  would  have  found  that  as  long 
ago  as  1815,  Herold  published  results  of  investiga- 
tions on  Papilio  brassica  and  other  species  of  Lepi- 
DOPTERA  which  proved  that  the  sex  of  the  larva  is 
already  determined  before  it  hatches  from  the  egg. 
A  similar  condition  was  reported  in  Bomhyx  pini  by 
Suckow  (1828),  in  Zeuzera  oesculi  hy  Vessels  (1867), 
and  in  Pieris  brassica  by  Brandt  (1878). 


GERM   CELLS  IN  THE  ARTHROPODzV    119 

There  now  ensues  a  period  of  activity  during 
which  a  large  number  of  ovarian  tubules  develop 
in  the  female  and  testicular  follicles  appear  in  the 
male.  A  number  of  much  debated  problems  exist 
regarding  the  cellular  elements  within  the  ovaries 
and  testes  of  insects  —  problems  which  are  of  con- 
siderable importance  in  any  discussion  of  the  germ- 
cell  cycle.  Put  in  the  form  of  questions,  two  of  these 
are  with  respect  to  the  ovary:  (1)  Do  the  nurse 
cells  originate  from  the  oogonia,  thus  becoming 
abortive  eggs,  or  are  they  of  mesodermal  parentage  ? 
(2)  Does  amitotic  nuclear  division  occur  in  nurse 
cells  and  oogonia  ? 

The  answers  to  these  questions  differ  according  to 
the  species  of  insects  studied,  and,  as  usual,  the  ob- 
servations and  interpretations  of  different  investi- 
gators do  not  always  agree.  They  can  be  answered 
with  certainty  in  the  case  of  Miastor.  All  of  the 
oogonia  in  this  form  are  direct  descendants  of  the 
primordial  germ  cell ;  the  nurse  cells  are  of  meso- 
dermal origin;  and  amitotic  division  occurs  neither 
in  the  nurse  cells  nor  in  the  oogonia.  The  situation 
is  quite  different  in  chrysomelid  beetles.  The  nurse 
cells  in  the  ovaries  of  the  potato  beetle  all  seem  to  be 
of  germ-cell  origin.  That  the  nurse  cells  which  are 
derived  from  oogonia  are  abortive  eggs  is  the  general 
opinion  of  zoologists.  Convincing  evidence  for  this 
view  has  recently  been  provided  by  De  Winter 
(1913)  from  studies  of  the  apterous  insect,  Podura 
aquatica.  In  this  species  the  proportion  of  eggs  and 
nurse  cells  which  develop  from  the  oocytes  is  about 


120        GERM-CELL   CYCLE   IN  ANIMALS 

one  to  ten.  The  oocytes  that  become  eggs  are  those 
that  chance  to  He  at  the  periphery  of  the  ovary  and 
hence  are  in  a  position  to  derive  abundant  nutrition 
from  the  blood.  The  oocytes  that  fail  to  become 
eggs  are  not,  according  to  De  Winter,  "  vitello- 
genes"  but  true  abortive  eggs,  representing  a  more 
primitive  stage  than  the  nurse  cells  of  other  insects 
which  have  acquired,  secondarily,  a  nutritive  func- 
tion. 

On  the  other  hand,  Govaerts  (1913)  argues  strongly 
in  favor  of  the  view  that  the  oogonia  divide  differen- 
tially, the  daughter  cells  becoming  true  germ  cells 
(the  ultimate  oogonia)  and  true  somatic  cells  (the 
nurse  cells).  He  bases  his  position  upon  the  condi- 
tions existing  in  the  ovaries  of  certain  beetles  of  the 
genera  Carabus  and  Cicitidela,  and  upon  the  dis- 
coveries of  Giardina  (1901),  Debaisieux  (1909), 
and  Gunthert  (1910)  in  Dytiscus  marginalis.  Giar- 
dina established  for  Dytiscus  the  fact  that  the  mito- 
ses which  result  in  the  formation  of  nurse  cells  are 
differential,  as  theoretically  postulated  by  Paulcke 
(1900).  During  the  four  divisions  preceding  the 
formation  of  the  oocyte  a  single  oogonium  gives  rise 
to  one  oocyte  and  fifteen  nurse  cells  (Fig.  38).  A 
differentiation  takes  place  in  the  chromatin  of  the 
oogonial  nucleus,  one  half  consisting  of  a  condensed 
mass,  the  other  half  of  large  granules  which  corre- 
spond to  the  forty  chromosomes  of  the  oogonium 
(Fig.  38,  A).  During  mitosis  the  chromosomes 
become  arranged  as  an  equatorial  plate,  and  the 
chromatic  mass  forms  a  ring  about  it  —  the  "anello 


GERM   CELLS   LN   THE   ARTHROPODA     121 


cromatico"  (B).     This  ring  passes  intact  to  one  of 
the  daughter  cells  (C),  whereas  the  chromosomes  are 


Fig.  38.  —  Differentiation  of  nurse  cells  and  oocytes  in  Dytiscus  inar- 
ginalis.  A.  Oogonium  with  chromatin  of  nucleus  separating  into 
two  parts.  B.  Metaphase  of  oogonial  mitosis;  the  "  anello  croma- 
tico" is  situated  at  the  lower  end  of  spindle.  C.  Two-cell  stage; 
the  lower  cell  with  nucleus  containing  two  sorts  of  chromatin. 
D.  Four-cell  stage;  "anello  cromatico"  in  one  cell.  E.  Eight-cell 
stage ;  cells  ready  to  divide.  F.  Sixteen-cell  stage  ;  one  large  cell 
(oocyte)  with  chromatin  from  the  "anello  cromatico,"  and  fifteen 
-     nurse  cells.    {A-D,  F,  from  Giardina,  1901;  E,  from  Dchaiseaux,  1909.) 

equally  divided.  During  the  succeeding  mitoses 
similar  differential  divisions  occur  result  in"'  in  one 
oocyte  containing  the  chromatic  ring  (Fig.  38,  F)  and 


122        GERM-CELL   CYCLE   IN  ANIMALS 

fifteen  nurse  cells  lacking  this  nuclear  substance. 
Thus  as  Paulcke's  theory  demands,  the  difference 
between  the  nurse  cells  and  the  oocytes  is  the  result 
of  internal  and  not  external  causes. 

Giardina  considered  the  formation  of  the  chromatic 
ring  as  a  sort  of  synapsis,  and  later  (1902)  distin- 
guished between  a  complete  synapsis,  such  as 
ordinarily  occurs  in  the  germ-cell  cycle,  and  a  partial 
synapsis  as  exhibited  by  Dytiscus.  Regarding  the 
significance  of  this  differential  mitosis,  he  maintains 
that  this  phenomenon  is  the  cause  of  the  differen- 
tiation into  nurse  cells  and  oocytes,  resulting  in  a 
complete  amount  of  chromatin  in  the  keimbahn 
cells  and  perhaps  also  an  unequal  distribution  of  cyto- 
plasmic substances.  As  in  the  case  of  Ascaris  and 
Miastor,  it  might  better  be  regarded  as  a  means  of 
depriving  the  nurse  cells  of  part  of  their  chromatin. 
Moreover,  Boveri  (1904)  has  compared  the  chroma- 
tin-diminution  in  Ascaris  with  Giardina's  differ- 
ential mitoses.  Debaisieux  (1909)  and  Gunthert 
(1910)  have  confirmed  Giardina's  results,  and  the 
latter  studied  two  other  Dytiscid^,  Acilius  and 
Colymhetes,  which  also  exhibit  differential  mitoses 
similar  except  in  certain  details.  Gunthert  found 
that  the  chromatic  ring  is  composed  of  fine  granules 
which  may  split  off  from  the  surface  of  the  chromo- 
somes (compare  with  Ascaris  and  Miastor)  and  stain 
like  cytoplasm.  He  interprets  this  as  "  Zerfallspro- 
dukte"  of  the  chromosomes.  Debaisieux,  on  the 
other  hand,  claims  that  this  cast-out  nuclear  material 
is  nucleolar  rather  than  chromatic  in  nature. 


GERM   CELLS   IN   THE   ARTHROPODA     123 

It  seems  highly  probable  that  the  "anello  croma- 
tico"  of  Giardina  consists  of  chromatin,  and  Gold- 
schmidt  (1904)  and  others  do  not  hesitate  to  class 
it  as  an  example  of  a  "  Chromidialapparat."  Further- 
more it  is  apparently  the  result  of  a  chromatin- 
diminution,  as  Boveri  (1904)  maintains,  differing 
from  the  similar  process  in  Ascaris  and  Miastor  in 
details  but  not  in  the  ultimate  result.  Finally,  the 
discovery  of  this  peculiar  body  in  Dytiscus  adds  one 
more  argument  to  the  hypothesis  that  the  chromatin 
content  of  the  germ  cells  differs  from  that  of  the 
somatic  cells  quantitatively,  at  least  in  some  cases, 
and  perhaps  also  qualitatively. 

Many  are  the  bodies  that  have  been  homologized 
with  the  "  anello  cromatico"  of  Dytiscus.  Buchner 
(1909)  claims  that  the  nucleolar-like  structure  in 
the  oogonia  and  young  oocytes  of  Gryllus  is  homol- 
ogous to  both  accessory  chromosomes  of  the  sper- 
matogenesis and  to  this  chromatin  ring  in  Dytiscus. 
This  "  accessorische  Korper"  passes  intact  into  one 
half  of  the  oocytes  where  it  disintegrates  into  granules 
of  a  "tropische  Natur."  Foot  and  Strobell  (1911) 
have  also  compared  it  with  the  chromatin  nucleolus 
in  the  oogonia  of  Protenor  with  which  it  has  certain 
characteristics  in  common,  but  no  such  differential 
divisions  occur  as  in  Dytiscus. 

Govaerts  (1913)  was  unable  to  find  anything 
resembling  the  chromatic  ring  of  Giardina,  and  con- 
cludes that  the  formation  of  a  chromatic  mass  dif- 
ferentiating the  oocytes  and  the  nurse  cells  is  unique 
in  the  Dytiscid^.     His  investigations  demonstrate 


124        GERM-CELL   CYCLE   IN  ANIMALS 

that  this  phenomenon  does  not  occur  in  all  insects  and 
that  we  must  seek  some  larger  cause  than  the  un- 
equal distribution  of  chromatic  elements. 

If  no  differential  divisions  are  present,  as  in 
Dytiscus,  what  is  the  cause  of  the  formation  of 
oocytes  and  nurse  cells  ?  Govaerts  decides  that  since 
the  ultimate  oogonium  possesses  a  definite  polarity 
marked  by  the  localization  of  the  "residu  fusorial," 
and  the  two  kinds  of  daughter  cells  arise  from  op- 
posite ends  of  the  mother  cell,  the  cause  of  the  differ- 
entiation resides  in  the  polarization  of  the  oogonium. 
He  does  not,  however,  account  for  this  "  polarite  pre- 
differentielle." 

Haecker  (1912)  has  described  in  Cyclops  and 
Diaptomus  a  three-cell  stage  in  the  development 
of  the  gonad  which  is  brought  about  by  the  delayed 
division  of  one  of  the  germ  cells  of  the  two-cell 
stage,  and  concludes  that  as  in  Dytiscus  there  must 
be  an  internal  difference  in  the  cells  to  account 
for  this  condition. 

Wieman  (19106)  has  followed  the  history  of  the 
oogonia  in  Leptinotarsa  signaticollis  through  the 
larval  and  adult  stages,  but  was  unable  to  find  any 
evidence  that  the  nuclei  inaugurate  differentiation 
as  in  Dytiscus.  He  concludes  that  "the  process 
seems  to  be  the  result  of  several  distinct  cell  elements 
which  operate  together  as  a  whole"  (p.  148)  and  that 
the  semi-fluid  matrix  which  results  from  the  lique- 
faction of  cells  at  the  base  of  the  terminal  chamber 
may  exert  a  "  specific  effect  on  those  germ  cells 
coming  under  its  influence,  enabling  them  to  develop 


GERM   CELLS   IN   THE   ARTHROPOD  A     125 

into  ova,  while  the  more  distant  germ  cells  become 
nurse  cells"  (p.  147).  My  observations  agree 
with  those  of  Wieman ;  no  definite  relations  nor 
nuclear  evidence  were  discovered  during  the  differ- 
entiation of  the  oogonia  into  oocytes  and  nurse  cells. 
The  data  available  do  not  suggest  any  method  of 
differentiation  not  already  proposed,  and  still  leave 
the  question  whether  the  nurse  cells  should  be 
regarded  as  abortive  germ  cells  or  true  somatic 
cells  one  of  personal  opinion. 

A  study  of  cyst  formation  in  the  testis  of  the  potato 
beetle  has  revealed  what  seems  to  be  a  series  of  events 
in  the  male  germ-cell  cycle  parallel  to  that  in  the 
females  of  Dytiscus,  Carabus,  and  Cicindela,  during 
which  the  nurse  cells  are  produced.  There  are  in 
Leptinotarsa  two  pairs  of  testes,  one  on  either  side  of 
the  body.  Each  testis  consists  of  a  large  number  of 
follicles  radiating  out  from  near  the  center.  Figure 
39  is  a  diagram  of  a  longitudinal  section  made 
from  the  testis  of  an  old  larva.  At  the  lower  end 
is  attached  the  sperm  duct  (s.d)  which  is  con- 
nected with  a  cavity  (c)  within  the  testis.  Just 
above  this  cavity  is  a  region  containing  degenerating 
cells ;  above  this  region  is  a  mass  of  spermatogonia 
(sg)  not  yet  within  cysts ;  and  this  mass  is  capped 
by  a  small  group  of  epithelial  cells  (t.c).  The  major 
part  of  the  testis  is  composed  of  radiating  follicles 
containing  cysts  of  spermatogonia,  spermatocytes, 
or  spermatozoa  (cy) . 

In  that  region  of  the  testis  surrounding  and  under- 


126        GERM-CELL  CYCLE   IN  ANIMALS 


lying  the  terminal  cap  (Fig.  39,  t.  c)  there  are  a  large 
number  of  spermatogonia  not  yet  contained  in  cysts. 
All  stages  in  cyst  formation  may  be  observed  here 
not  only  in  larval  testes  but  also  in  those  of  pupse 
and  adults.  The  youngest  spermatogonia  are  those 
lying  near  the  terminal  cap.     Figure  40,  A  shows  a 

few  cells  of  the 
terminal  cap  {t.c), 
some  of  the  neigh- 
boring spermato- 
gonia {sjpg) ,  and 
several  of  the  epi- 
thelial cells  {ep) 
that  are  scattered 
about  among  the 
spermatogonia. 
V  vsts  fj re  Tormed 

Fig.   39. — Leptinotarsa    decemlincata.     Longi-  "^ 

tudinal  section  through  testis  of  full-grown  toward    the     edge 
larva.       c  =  cavity;    c?/ =  region    of    cysts;  e:    x.\^                         x. 
s.d  =  sperm  duct;  s^  =  region  of  spermato-  ^'-    ^^^    Spermato- 
gonia ;    sv  =  region    of   spermatozoa ;    t.c  =  gonial  maSS  away 
terminal  cap.  °                                  .      '^ 

from  the  terminal 
cap,  and  Fig.  40,  A  to  G  represent  certain  of  the 
stages  observed.  The  spermatogonia  divide  ap- 
parently exclusively  by  mitosis.  A  well-developed 
spindle  is  formed  and  this  persists  after  the  cell  wall 
has  separated  the  two  daughter  cells.  The  spindle 
fibers  which  are  at  first  perfectly  distinct  (Fig.  40,  B) 
unite  into  a  compact  strand  (Fig.  40,  C)  which 
stains  dense  black  in  iron  hsematoxylin  after  fixa- 
tion in  Carnoy's  fluid.  In  many  cases  it  was  im- 
possible   to    distinguish    an    intervening    cell    wall 


D 


Fig    40  _r     ,.  ^  ^ 

^-       ^^Vi^notarsa  decemlineata      qfn„     • 

in  ^amo  process      n'"R''"'f"  »'  ^P"n,atoSun,^'  r""!  T'""'"'" 
lial    envdopepp,    """■''■'•'*''  «P<'™atogo„iarocJl  wi,^     '''";'"' 

<?•  S^^*'»  through  eysfeomairg'thStr'""'""'^    '''''"    '-"  • 

g  Mirty-two  spormatogouia.  (127) 


128        GERM-CELL   CYCLE   IN  ANIMALS 

between  the  daughter  nuclei  (Fig.  40,  D).  In 
either  case,  however,  the  spindle  remains  persist, 
forming  a  basic  staining  strand  with  enlarged  ends 
connecting  the  two  nuclei.  Since  at  this  time  and 
in  all  later  stages  the  two  or  more  spermatogonia  may 
be  found  surrounded  by  an  envelope  of  epithelial 
cells,  it  seems  certain  that,  as  Wieman  (19106) 
maintains,  the  spermatozoa  in  a  single  cyst  are 
derived  from  a  single  spermatogonium. 

A  cyst  containing  four  spermatogonia  is  repre- 
sented in  Fig.  40,  E.  Here  again  appear  the  strongly 
basic  staining  spindle  remains  connecting  the  nuclei. 
These  black  strands  persist  until  the  succeeding 
mitotic  division  occurs  as  Fig.  40,  F,  which  was 
drawn  from  a  section  of  a  cyst  containing  eight 
spermatogonia,  shows.  Spindle  remains  are  still 
evident  in  later  stages,  as  in  Fig.  40,  G,  which  repre- 
sents a  cyst  containing  thirty-two  spermatogonia,  but 
were  not  observed  in  cysts  containing  more  than 
sixty-four  cells. 

Many  investigators  have  figured  spermatogonial 
divisions  which  result  in  rosette-like  groups  of  cells 
similar  to  that  represented  in  Fig.  40,  F.  Ap- 
parently, however,  the  spindle  remains,  if  present,  did 
not  possess  such  a  strong  affinity  for  basic  stains. 
Furthermore,  only  those  of  my  preparations  that 
were  fixed  in  Carnoy's  fluid  and  stained  in  iron 
hsemotoxylin  exhibited  these  black  strands.  Similar 
spindle  remains  have  been  observed  in  Dytiscus, 
especially  by  Gunthert  (1910),  and  Carabus  (Go- 
vaerts,    1913),    during   the   differentiation   of   nurse 


GERM  CELLS  IN  THE  ARTHROPODA     129 

cells  and  oocytes  from  oogonia,  and  there  can  be 
little  doubt  but  that  the  process  of  cyst  formation 
in  the  male  as  described  above  is  similar  to  the  differ- 
ential divisions  in  the  female. 

Thus  the  discovery  of  these  distinct  spindle  re- 
mains in  the  spermatogonial  divisions  enables  us 
to  homologize  one  more  period  in  the  cycle  of  the 
male  germ  cells  with  a  corresponding  period  in  the 
cycle  of  the  female  germ  cells. 

According  to  this  view  the  ultimate  spermato- 
gonium passes  through  a  certain  number  of  divisions 
—  probably  five  or  six  —  which  correspond  to  the 
differential  divisions  so  clearly  exhibited  by  the 
ultimate  oogonia  of  Dytiscus.  Just  as  in  the  matura- 
tion processes,  however,  where  only  one  female  cell 
but  all  of  the  male  cells  are  functional,  so  these 
earlier  divisions  result  in  the  female  in  the  pro- 
duction of  a  single  oocyte  and  a  number  of  nurse 
cells  which  may  be  considered  abortive  eggs,  whereas 
in  the  male  every  daughter  cell  is  functional.  The 
limited  period  of  division  in  the  cycle  of  the  male 
germ  cells  in  man  (Montgomery,  1911 ;  von  Wini- 
warter, 1912)  is  also  similar  to  those  in  Dytiscus  and 
Leptinotarsa.  The  Sertoli  cells  are  intimately  con- 
nected with  the  germ  cells  in  the  mammalian  testis 
and  probably  perform  three  functions :  (1)  they 
nourish  the  spermatocytes ;  (2)  they  provide  the 
spermatic  fluid ;  and  (3)  they  exert  some  chemico- 
tactic  stimulus  which  serves  to  orient  the  spermato- 
zoa into  bundles.  The  origin  of  the  Sertoli  cells  has 
been  for  many  years  in  doubt.     Many  investigators 


130        GERM-CELL   CYCLE   IN  ANIMALS 

claim  that  they  arise  from  cells  other  than  germ  cells ; 
these  writers  have  been  called  by  Waldeyer  (1906) 
"  dualists."  An  equal  number  of  authorities  be- 
lieve   that    both    Sertoli    cells    and    spermatogonia 


Fig.  41.  —  Stages  in  the  formation  of  the  Sertoli  cell  in  man.  A.  Sper- 
matogonium containing  granular  inclusion  {X)  from  which  "  Sertoli 
cell  determinant  "  ma^/ arise.  B.  Antepenultimate  spermatogonium 
showing  rod  (R)  and  idiozome  (/).  C.  Division  of  rod.  D.  A  Ser- 
toli cell  containing  a  divided  rod  (R)  and  two  rodlets  {rz).  E.  Ser- 
toli cell  with  crystalloid  of  Charcot  and  lipoid  granules  ;  at  lower 
right  corner  a  spermatogonium  with  crystalloid  of  Lubarsch.  {A—D, 
from  Montgomery,  1911;  E,  from  von  Winiwarter,  1912.) 

originate  from  primordial  germ  cells ;    these  are  the 
"  monists." 

The  researches  of  Montgomery  and  von  Wini- 
warter have  decided  the  question,  at  least  so  far  as 
man  is  concerned,  in  favor  of  the  monists.  Mont- 
gomery's results  are  diagrammatically  shown  in 
Fig.  42.  Of  thirty  antepenultimate  spermatogonia 
examined,  twenty-three  contained  each  a  rod-shaped 
structure  (Fig.  41,  B,  R)  and  it  seems  probable 
that  this  peculiar  body,  which  is  identified  by  von 
Winiwarter  with  the  "  cristalloide  de  Lubarsch " 
(Lubarsch,   1896),  is  present  in  every  cell   of  this 


GERM   CELLS   IN   THE   ARTHROPODA     131 

generation.  This  rod  is  considered  by  Montgomery 
to  be  of  cytoplasmic  origin  and  is  termed  by  him  a 
"  Sertoli    cell    determinant."     During    the   division 


Antepenultimate 
Spermatogonium 


Penultimate 
Spermatogonia 


Ultimate      _  / 
Spermatogonia 


Sertoli  y" 
cell     \ 


Germ  Cells  Proper 


Fig.  42.  —  Diagram  illustrating  the  differentiation  of  the  Sertoli  cell  in 

man.     {From  Montgomery,  1911.) 

of  the  antepenultimate  spermatogonia  the  rod  passes 
undivided  into  one  of  the  daughter  cells ;  thus  one- 
half   of   the   penultimate   spermatogonia   possess   a 


132        GERM-CELL   CYCLE   IN  ANIMALS 

rod,  the  other  half  do  not.  Of  the  forty-nine  penul- 
timate spermatogonia  examined,  twenty-four  ex- 
hibited a  rod  and  twenty-five  did  not.  This  result 
has  been  confirmed  by  von  Winiwarter.  When  the 
rod-containing  penultimate  spermatogonia  divide, 
there  is  a  similar  segregation  of  the  rod  in  one  of 
the  daughter  cells,  hence  only  one-fourth  of  the  cells 
resulting  from  the  divisions  of  the  antepenultimate 
spermatogonia  possess  a  rod.  Of  one  hundred 
and  forty-two  cells  of  this  generation  studied  by 
Montgomery,  twenty-five  were  found  with  a  rod  and 
one  hundred  and  seventeen  without.  That  this 
ratio  is  less  than  one  to  three  (1:3)  is  explained  by 
the  fact  that  some  of  the  spermatogonia  with  rods 
may  already  have  become  Sertoli  cells.  The  further 
history  of  the  rod  in  the  Sertoli  cell  is  as  follows  :  A 
primary  rodlet  is  produced  by  a  splitting  of  the  rod 
(Fig.  41,  C)  after  which  the  rod  either  disappears 
at  once  or  else  persists  for  a  time,  in  which  case  it 
may  split  longitudinally  as  shown  in  Fig.  41,  D. 
However,  in  four-fifths  of  the  cells  examined  (one 
hundred  in  number)  the  large  rod  disappeared 
before  the  growi;h  of  the  Sertoli  cell  had  begun. 
Each  primary  rodlet  splits  longitudinally  into  two 
approximately  equal  parts,  called  secondary  rodlets 
(Fig.  41,  Z),  r2),  which  persist  until  the  end  of  the 
cycle  of  the  Sertoli  cell. 

Neither  Montgomery  nor  von  Winiwarter  were 
able  to  determine  the  origin  of  the  rod.  They  do 
not  consider  it  mitochondrial  in  nature,  although 
it  may  arise  from  granules  lying  in  the  cytoplasm. 


GERM   CELLS   IN   THE   ARTHROPOD  A     133 

Montgomery  found  in  one  cell  a  mass  of  granules 
from  which  the  rod  may  have  developed  (Fig.  41,  yl, 
X),  and  von  Winiwarter  noted  that  the  rod  had  a 
granular  appearance  in  the  earliest  stages  he  ex- 
amined. It  is  also  perfectly  distinct  from  the  io- 
zome  (see  Fig.  41,  B,  I)  and  is  apparently  not 
directly  derived  from  the  nucleus.  Von  Winiwarter 
is  not  so  certain  as  Montgomery  regarding  the  history 
of  the  spermatogonia,  the"  cristalloide  de  Lubarsche," 
and  the  "  batonnets  accessoires,"  as  he  calls  the 
rodlets.  He  was  unable  to  decide  regarding  the 
number  of  spermatogonial  divisions  and  believes  it 
to  be  indeterminate.  He  finds,  contrary  to  Mont- 
gomery, the  rod  persisting  in  fully  developed  Sertoli 
cells,  and  considers  the  fragmentation  or  fission  of 
the  rod  to  form  the  primary  rodlets  as  doubtful. 
Further  investigations  with  more  favorable  material 
are  very  desirable,  but  notwithstanding  certain 
differences  of  opinion  between  the  two  writers  whose 
results  have  been  briefly  stated  above,  it  seems  cer- 
tain that  Sertoli  cells  and  germ  cells  are  both  derived 
from  primordial  germ  cells,  and  that  the  Sertoli 
cells  differ  from  the  ultimate  spermatogonia  in  the 
possession  of  a  peculiar  rod  probably  of  cytoplasmic 
origin.  Montgomery  considers  this  a  sort  of  secon- 
dary somatic  differentiation  (the  Sertoli  cells  repre- 
senting the  soma  of  the  testis) ;  the  first  somatic  dif- 
ferentiation occurring  when  the  tissue  cells  become 
differentiated  from  the  germ  cells  in  the  embryo. 

Amitosis.     Wilson     (1900)     defines    amitosis    as 
"  mass-division    of    the  nuclear   substance    without 


134        GERM-CELL   CYCLE   IN  ANIMALS 

the  formation  of  chromosomes  and  amphiaster" 
(p.  437)  and  concludes  from  a  review  of  the  literature 
up  to  the  year  1900  "that  in  the  vast  majority  of 
cases  amitosis  is  a  secondary  process  which  does  not 
fall  in  the  generative  series  of  cell-divisions"  (p.  119). 
During  the  past  ten  years  interest  in  direct  nuclear 
division  has  been  maintained  principally  because  of 
the  claims  of  certain  investigators  that  germ  cells 
may  multiply  in  this  way  and  still  give  rise  to  func- 
tional eggs  or  spermatozoa. 

During  amitosis  the  chromatin  remains  scattered 
within  the  nucleus  and  does  not  form  a  spireme 
nor  chromosomes,  and  therefore  its  individual  ele- 
ments, the  chromatin  granules,  do  not  divide.  As 
a  result  of  this  ma^^-division  there  can  be  no  accurate 
segregation  of  chromatin  granules  in  the  daughter 
nuclei  as  is  demanded  by  the  theory  that  the  nucleus, 
and  particularly  the  chromatin,  contains  the  de- 
terminers of  hereditary  characteristics.  Further- 
more, nuclear  division  without  the  formation  of 
chromosomes  obviously  condemns  the  hypothesis 
of  the  genetic  continuity  of  the  chromosomes,  and 
hence  seriously  interferes  with  current  ideas  regard- 
ing the  significance  of  the  accessory  chromosomes  in 
the  determination  of  sex.  Among  the  animals  in 
whose  germ  cells  amitosis  has  been  reported  are  cer- 
tain Amphibia,  coelenterates,  cestodes,  and  insects. 

Amphibia.  Vom  Rath  (1891,  1893),  Meves  (1891, 
1895),  and  McGregor  (1899)  have  recorded  amitosis 
in  the  germ  cells  of  Amphibia.  Meves  claims  that 
the  spermatogonia  of  Salamandra  divide  amitotically 


GERM   CELLS   IN   THE   ARTHROPODA     135 

in  the  autumn  but  return  to  the  mitotic  method  in 
the  spring,  later  giving  rise  to  functional  spermato- 
gonia. Vom  Rath  finds  amitosis  but  contends  that 
the  cells  that  divide  in  this  way  do  not  become  sper- 
matozoa but  are  degenerating,  being  used  as  nutritive 
material  by  the  other  spermatogonia.  The  amitotic 
divisions  described  by  McGregor  (1899)  in  Aiii- 
phiurna  differ  in  certain  respects  from  those  of 
both  Meves  and  vom  Rath.  In  this  species  the 
primary  spermatogonia  divide  by  amitosis;  their 
products  later  divide  by  mitosis  and  produce  func- 
tional spermatozoa.  Our  knowledge  concerning  ami- 
tosis in  the  spermatogonia  of  Amphibia  is  therefore 
in  an  unsatisfactory  state,  although  the  observations 
of  Meves  and  McGregor  argue  strongly  in  favor  of 
this  method. 

CcELENTERATA.  While  uo  direct  nuclear  divi- 
sions were  recorded  by  Hargitt  (1906)  in  the  germ 
cells  of  Clava  leptostyla  the  absence  of  mitotic  figures 
in  the  early  cleavage  stages  of  the  egg  led  him  to  the 
conclusion  that  the  "nuclear  activity  differs  greatly 
from  the  oridinary  forms  of  mitosis,  and  appears 
to  involve  direct  or  amitotic  division"  (p.  229). 
If  this  were  true,  the  germ  cells  which  are  derived 
from  these  cleavage  cells  must  be  descended  from 
cells  which  once  divided  amitotically.  This  case 
of  supposed  amitosis  has  been  cleared  up  by  the  sub- 
sequent studies  of  Beckwith  (1909),  who  collected 
material  of  Clava  very  early  in  the  morning  and  found 
typical  mitotic  divisions  during  the  maturation  and 
early  cleavage  of  the  egg  and  no  evidence  of  amitosis. 


136        GERM-CELL   CYCLE   IN  ANIMALS 

Cestoda.  Child  concluded  (1904)  from  a  study 
of  the  cestode,  Moniezia  expansa,  that  this  method 
of  cell  division  occurs  in  the  antecedents  of  both 
the  eggs  and  the  spermatozoa.  This  writer  has 
published  a  series  of  papers  upon  this  subject  using 
Moniezia  expansa  and  Moniezia  planissima  for  his 
material  (1904,  1906,  1907,  1910,  1911),  and  his 
principal  conclusion  is  that  in  these  species  the 
division  of  the  cells  destined  to  become  eggs  and 
spermatozoa  is  predominantly  amitotic.  Mitotic 
division  also  occurs  but  comparatively  rarely.  Cells 
which  have  divided  amitotically  then  divide  mitoti- 
cally  during  maturation  and  form  typical  ova. 

The  nature  of  the  nuclear  division  in  the  cestodes 
was  later  investigated  by  Richards  (1909,  1911)  who 
studied  the  female  sex  organs  of  the  same  species 
employed  by  Child  as  well  as  material  obtained  from 
Tcenia  serrata.  Richards  finds  that  mitosis  unques- 
tionably occurs  in  the  young  germ  cells  but  was 
unable  to  demonstrate  amitosis.  Richards  claims 
that  amitosis  cannot  be  demonstrated  except  by  the 
observation  of  the  process  in  the  living  material  and 
the  subsequent  study  of  this  material  by  cytolog- 
ical  methods.  Child  (1911)  agrees  with  Richards 
that  amitosis  cannot  be  demonstrated  in  fixed 
material  but  nevertheless  concludes  after  an  examina- 
tion of  Richards'  preparations  *'that  direct  division 
plays  an  important  part  in  the  developmental  cycle 
of  Moniezia,  in  the  germ  cells  as  well  as  in  the  soma  " 
(Child,  1911,  p.  295). 

Finally  Harman   (1913)   was  unable  to  find  any 


GERM   CELLS   IN   THE   ARTHROPOD  A     137 

evidence  of  amitotic  divisions  in  the  sex  cells  of  either 
Tcenia  tenioeformis  or  Moniezia  and  concludes  that 
the  conditions  that  suggest  amitosis  can  just  as  well 
or  better  be  explained  by  mitosis.  Experiments 
with  living  cells  of  Tcenia  were  without  results, 
since  the  cells  did  not  divide  when  placed  in  Ringer's 
solution,  although  they  continued  to  live  outside  the 
body  of  the  host  for  forty-eight  hours.  Morse 
(1911)  likewise  failed  to  observe  divisions  in  living 
cells  of  Calliohothrium  and  Crossohothrium  which 
were  kept  in  the  plasma  of  the  host.  That  the 
observation  of  amitosis  in  living  cells  is  possible 
seems  certain  since  Holmes  (1913)  has  recorded  an 
actual  increase  in  the  number  of  epithelial  cells 
from  the  embryos  and  young  tadpoles  of  several 
Amphibia  that 'were  cultivated  in  lymph,  and  has 
noted  various  stages  of  amitotic  nuclear  division, 
although  no  convincing  evidence  was  obtained  that 
this  was  followed  by  cell  division. 

Insecta.  In  the  Hemiptera  amitosis  was  de- 
scribed by  Preusse  (1895)  in  the  ovarian  cells  of 
Nepa  cinerea  and  similar  conditions  were  reported 
by  Gross  (1901)  in  insects  of  the  same  order.  Gross, 
however,  claims  that  the  cells  which  divide  amitoti- 
cally  do  not  produce  ova  but  are  degenerating  or 
secretory. 

Foot  and  Strobell  (1911)  described  in  ovaries  of 
the  bug,  Protenor,  the  amitotic  division  of  certain 
cells  which  later  produce  ova.  There  is,  however, 
considerable  difference  of  opinion  among  investi- 
gators as  to  the  origin  of  the  ova  from  the  various 


138        GERM-CELL   CYCLE   IN  ANIMALS 

regions  of  the  insect  ovary  and,  since  Payne  (1912) 
has  shown  that  in  Gelastocoris  the  cells  that  appar- 
ently multiply  amitotically  do  not  produce  ova,  it 
seems  safe  to  conclude  that  in  Protenor  the  ova  are 
not  descended  from  cells  that  divide  amitotically. 

Amitotic  division  of  germ  cells  followed  by  mitotic 
division  has  been  described  by  Wieman  (19106, 
1910c)  in  the  ovaries  and  testes  as  well  as  in  the  nurse 


Fig.  43.  —  Stages  in  amitosis  in  spermatogonium  of  Leptinotarsa  signa- 

ticollis.     {From  Wieman,  1910.) 

cells  of  a  chrysomelid  beetle,  Leptinotarsa  signati- 
collis.  Germ  cells  in  both  ovary  and  testis  taken 
from  full-grown  larvae  were  found  in  stages  of  divi- 
sion recognized  by  Wieman  as  amitotic  (Fig.  43). 
It  was  difficult  to  demonstrate  actual  division  of 
the  cytoplasm,  but  that  such  a  division  really  occurs 
was  inferred  because  binucleated  cells  apparently 
gave  rise  to  spermatocytes  with  single  nuclei.  Rapid 
cell  division  is  assumed  by  Wieman  to  account  for 
amitosis.  This  is  brought  about  by  fluctuations  in 
the  nutritive  supply  or,  in  the  case  of  the  testis,  by 
the  rapid  proliferation  of  cells  during  the  formation  of 
cysts. 


GERM   CELLS   IN   THE   ARTHROPODA     139 

I  have  studied  my  preparations  of  chrysomelid 
beetles  carefully  with  the  aim  of  detecting  amitotic 
division  and  have  observed  what  appears  to  be  direct 
nuclear  division  among  the  nurse  cells,  but  could  not 
demonstrate  with  certainty  this  kind  of  division 
among  the  oogonia,  or  spermatogonia.  Three  stages 
in  the  direct  division  of  nurse  cell  nuclei  in  Leptiiiotarsa 
decemlineata  are  shown  in  Fig.  8,  a-c.  Oogonia 
and  spermatogonia,  however,  do  not  exhibit  such 
clearly  defined  stages  and  after  examining  my  prep- 
arations and  several  slides  kindly  sent  me  by  Doctor 
Wieman  I  am  forced  to  conclude  that  amitosis  has 
not  been  demonstrated.  It  is  true  that  frequently 
dumb-bell  shaped  nucleoli  occur  in  certain  of  the 
nuclei  and  frequently  two  nucleoli  are  present  at 
opposite  ends.  Also  two  nuclei  may  be  surrounded 
by  a  single  cell  wall,  but  no  stages  were  present  which 
could  not  be  attributed  as  well  or  better  to  mitotic 
phenomena. 

Conclusion.  From  the  evidence  at  present 
available  we  must  conclude  that  amitotic  division 
of  the  germ  cells  has  not  been  demonstrated,  and 
that  not  until  such  a  process  is  actually  observed 
in  living  cells  will  any  other  conclusion  be  possible. 

There  are  still  two  questions  regarding  the  germ- 
cell  cycle  in  beetles  that  we  shall  attempt  to  answer ; 
(1)  Does  a  chromatin-diminution  process  occur 
such  as  has  been  described  in  Miastor  and  Ascaris? 
and  (2)  Is  the  segregation  of  the  germ  cells  controlled 
by  the  nuclei  or  by  the  cytoplasm  ? 

The  fact  that  part  of  each  chromosome  is  cast  out 


140        GERM-CELL   CYCLE   IN  ANIMALS 

into  the  cytoplasm  in  all  except  the  "stem-cell" 
during  the  early  cleavage  of  Ascaris  is  well  known 
(see  p.  174,  Fig.  51).  A  similar  process  was  described 
by  Kahle  (1908)  in  Miastor  metraloas  and  confirmed 
by  me  (Hegner,  1912,  1914a)  in  Miastor  americana 
(see  p.  57,  Fig.  16).  This  chromatin-diminution 
process  results  in  the  formation  of  a  single  primordial 
germ  cell  containing  the  complete  amount  of  chroma- 
tin and  a  number  of  somatic  cells  with  a  reduced 
amount  of  chromatin.  The  origin  of  the  germ  cells 
has  been  carefully  studied  in  a  number  of  forms  which 
in  other  respects  resemble  Ascaris  and  Miastor,  but 
in  none  of  them  has  such  a  process  been  discovered. 
Hasper  (1911)  was  unable  to  establish  it  for  Chirono- 
mus  which  is  very  similar  to  Miastor  in  early  develop- 
ment, nor  has  such  a  phenomenon  been  found  in  Sagitta 
(Elpatiewsky,  1909,  1910 ;  Stevens,  19106;  Buchner, 
1910a,  19106)  and  the  copepods  (Haecker,  1897; 
Amma,  1911)  and  Cladocera  (Kuhn,  1911,  1913) 
which  undergo  total  cleavage  and  are  in  certain 
other  respects  similar  to  Ascaris. 

The  nuclear  divisions  in  the  eggs  of  chrysomelid 
beetles  have  been  examined  by  the  writer  with  con- 
siderable care,  but  nothing  resembling  a  diminution 
process  was  found.  Furthermore,  there  are  no 
evidences  of  chromatin  bodies  in  the  cytoplasm  or 
yolk  as  in  Ascaris  (Fig.  51)  and  Miastor  (Fig.  18,  cR), 
where  the  cast-out  chromatin  does  not  disintegrate 
immediately,  but  can  be  distinguished  for  a  consider- 
able period  during  early  embryonic  development. 
It   seems   necessarv   to   conclude  therefore  that  in 


GERM   CELLS   IN   THE   ARTHROPODA     141 

chrysomelid  eggs  both  germ  cells  and  somatic  cells 
possess  the  full  amount  of  chromatin  or  else  the 
elimination  of  this  substance  takes  place  in  some 
other  way. 

The  Differentiation  of  the  Nuclei  of  the 
Blastoderm  cells,  Primordial  Germ  Cells,  and 
ViTELLOPHAGS.  The  couclusiou  that  no  chromatin- 
diminution  process  occurs  during  the  early  cleav- 
age divisions  in  the  eggs  of  chrysomelid  beetles 
necessitates  the  search  for  some  other  method  of 
differentiation  among  the  cleavage  nuclei.  The 
insect  egg  is  particularly  advantageous  for  testing 
Roux's  hypothesis  of  qualitative  nuclear  division, 
since  we.  have  here  the  production  of  an  enormous 
number  of  nuclei  before  any  cell  walls  are  formed, 
and  an  egg  that  is  remarkably  definitely  organized, 
as  indicated  by  my  experiments  (Hegner,  19096, 
1911a),  before  the  blastoderm  is  formed. 

I  have  been  unable  to  find  any  differences  in  the 
nuclei  before  they  fuse  with  the  keimhautblastem, 
but  as  soon  as  this  does  occur,  a  gradual  change  takes 
place,  and  at  the  time  when  the  blastoderm  is  com- 
pleted three  sorts  of  nuclei  are  distinguishable: 
(1)  The  nuclei  of  the  primordial  germ  cells  (Fig.  36, 
C)  are  larger  than  the  others  and  contain  compara- 
tively few  spherical  chromatin  granules  evenly  dis- 
tributed. The  cytoplasm  of  these  cells  is  distin- 
guishable from  that  of  all  other  cells  because  of  the 
presence  of  granules  from  pole-disc.  (2)  The  nuclei 
of  the  blastoderm  cells  are  small  and  completely 
filled     with     large     spherical     chromatin     granules. 


142        GERM-CELL   CYCLE   IN  ANIMALS 

(3)  The  nuclei  of  the  vitellophags  resemble  the 
early  cleavage  nuclei ;  they  are  midway  between 
the  other  two  kinds  in  size,  and  their  chromatin  is  in 
a  more  diffuse  condition. 

Whether  these  three  kinds  of  nuclei  were  all 
potentially  alike  before  their  differentiation  is  an 
important  question.  Visibly  they  are  all  similar 
until  they  become  localized  in  definite  regions  of 
the  egg,  and  associated  with  particular  cytoplasmic 
elements.  One  cannot  help  but  conclude  that  they 
were  all  potentially  alike  and  that  their  differentia- 
tion was  brought  about  through  the  influence  of 
the  cytoplasm  in  which  they  happened  to  become 
embedded.  The  writer  has  shown  (Hegner,  1911a) 
that  if  the  posterior  end  of  a  freshly  laid  egg  of 
Leptinotarsa  decemlineata  is  killed  with  a  hot  needle, 
thus  preventing  the  pole-disc  granules  and  surround- 
ing cytoplasm  from  taking  part  in  development,  no 
primordial  germ  cells  will  be  produced.  A  large 
series  of  similar  experiments  have  also  proved  that 
at  the  time  of  deposition,  "  The  areas  of  the  peripheral 
layer  of  cytoplasm  (Fig.  36  khbl.)  are  already  set 
aside  for  the  production  of  particular  parts  of  the 
embryo,  and  if  the  areas  are  killed,  the  parts  of  the 
embryo  to  which  they  were  destined  to  give  rise 
will  not  appear.  Likewise,  areas  of  the  blastoderm 
are  destined  to  produce  certain  particular  parts  of 
the  embryo"  (Hegner,  1911a,  p.  251).  What 
becomes  of  the  nuclei  that  are  prevented  from  enter- 
ing the  injured  region  of  the  egg  ?  No  evidence 
has  been  discovered  to  indicate  that  they  disinte- 


GERM   CELLS   IN   THE   ARTHROPODA     143 

grate,  so  they  probably  take  part  in  development 
after  becoming  associated  with  some  other  part  of 
the  egg.  If  these  nuclei  were  qualitatively  different 
they  should  produce  germ  cells  and  other  varieties  of 
cells  in  whatever  region  they  chance  to  reach.  It 
is  evident  that  they  are  not  potentially  different 
and  that  their  "prospective  potency"  and  "pro- 
spective significance"  do  not  coincide.  The  cyto- 
plasm is,  therefore,  the  controlHng  factor  at  this 
stage  in  the  germ-cell  cycle,  although  cytoplasmic 
differentiations  are  for  the  most  part  invisible  and 
probably  the  result  of  nuclear  activity  during  earlier 

stages. 

Hymenoptera.  a  number  of  papers  have  ap- 
peared which  contain  references  to  the  germ  glands 
of  Hymenoptera  (Hegner,  1909,  pp.  245-248). 
The  most  important  of  these  from  the  standpoint  of 
the  present  discussion  are:  (1)  Silvestri  (1906,  1908) 
and  Hegner  (19145)  on  some  parasitic  species,  and 
(2)  Petrunkewitsch  (1901,  1903),  Nachtsheim  (1913), 
and  others  on  the  honey-bee. 

In  an  endeavor  to  test  the  "  Dzierzon  theory," 
that  the  eggs  which  produce  drone  bees  are  normally 
unfertilized,  Petrunkewitsch  (1901-1903)  discovered 
some  usual  maturation  divisions.  In  "  drone  eggs" 
the  first  polar  body  passes  through  an  equatorial 
division,  each  of  its  daughter  nuclei  containing  one- 
half  of  the  somatic  number  of  chromosomes.  The 
inner  one  of  these  daughter  nuclei  fuses  willi  the 
second  polar  body,  which  also  contains  one-half  of 
the  somatic  number  of  chromosomes ;   the  resultant 


144        GERM-CELL   CYCLE   IN  ANIMALS 

nucleus  with  sixteen  chromosomes,  the  "  Richtungs- 
kopulationskern "  passes  through  three  divisions, 
giving  rise  to  eight  "  doppelkernige  Zellen."  After 
the  blastoderm  is  completed,  the  products  of  these 
eight  cells  lie  in  the  middle  line  near  the  dorsal  surface 
of  the  egg,  where  the  formation  of  the  amnion  begins  ; 
the  nuclei  of  these  cells  are  small,  and  lie  embedded 
in  dark  staining  cytoplasm.  Later  they  are  found 
just  beneath  the  dorsal  surface  near  the  point  of 
union  of  the  amnion  with  the  head-fold  of  the  em- 
bryonic rudiment.  They  are  next  located  between 
the  epithelium  of  the  mid-intestine  and  the  ectoderm  ; 
from  here  they  migrate  into  the  coelomic  cavities, 
and  finally,  at  the  time  of  hatching,  form  a  "  wellen- 
artigen"  strand,  the  germ-gland,  extending  through 
the  third,  fourth,  fifth,  and  sixth  abdominal  segments. 
The  fertilized  eggs  of  the  bee  were  also  examined 
by  Petrunkewitsch,  but  no  *'  Richtungskopulations- 
kern"  was  discovered.  In  these  eggs  the  genital 
glands  arise  from  mesoderm  cells.  Doubt  was 
immediately  cast  on  these  results,  although  Weismann 
(1904,  p.  336)  vouched  for  their  accuracy.  Thus 
Wheeler  (1904)  says,  "  Even  in  his  first  paper  there  is 
no  satisfactory  evidence  to  show  that  the  cells  re- 
garded as  derivatives  of  the  polar  bodies  in  the  figures 
on  plate  4  are  really  such,  and  not  dividing  cleavage 
cells  or  possibly  vitellophags.  .  .  .  When  we  take 
up  the  second  paper  we  wonder  how  anybody  could 
regard  the  figures  there  presented  as  even  an  adum- 
bration of  proof  that  the  testes  of  the  drone  are  de- 
veloped   from    the    polar    bodies."     Dickel    (1904) 


GERM   CELLS   IN   THE   ARTHROPOD  A     145 

could  find  no  connection  between  the  polar  bodies  and 
the  cells  Petrunkewitsch  claims  originate  from  the 
"  Richtungskopulationskern,"  but  considers  these 
''  Dotterzellen."  Nachtsheim  (1913)  agrees  with 
Dickel,  that  these  are  yolk  cells  and  have  no  relation 
to  the  polar  bodies.  He  also  finds  these  cells  in  both 
fertilized  and  unfertilized  eggs,  not  as  Petrunkewitsch 
states  only  in  the  latter. 

The   investigations   of   Silvestri    (1906,    1908)    on 
parasitic   Hymenoptera   are   of   particular   interest, 
since  in  both  the  polyembryonic  species  and  those 
whose  eggs  produce  a  single  individual,  the  keimbahn- 
determinant   is    considered   by   him  to  represent  a 
plasmosome  which  escapes  from  the  germinal  vesicle. 
Silvestri  (1906,   1908)  has  described  the  embryonic 
development   of   both   monembryonic   and   polyem- 
bryonic  hymenopterous   parasites.     Of   the   former 
Encyrtus  aphidivorus  and  Oophthora  semhlidis  were 
studied ;    in  both  species  the  series  of  events  were 
found  to  be  similar.     The  egg  at  the  time  of  deposi- 
tion is  elongated  and  irregularly  oval  in  shape  (Fig. 
44,  A)  ;    it  contains  a  germinal  vesicle   (.1)   in  the 
anterior  region  and  a  deeply  staining  body  near  the 
posterior  end  which  is  called  by  Silvestri  the  "  nu- 
cleolo"   (N)  and  is  stated  to    be  derived   from    the 
nucleolus    of   the   oocyte   nucleus.     The   eggs    may 
develop   parthenogenetically    or    after   fertilization; 
the  unfertilized  eggs  produce  males,  whereas  the  fer- 
tihzed   eggs   develop   into   females.     In   either   case 
two  polar  bodies  are  produced;    these  disintegrate 
later.     The  cleavage  nucleus  produces  by  a   series 


146        GERM-CELL  CYCLE   IN  ANIMALS 


of  divisions  a  number  of  nuclei  which  migrate  to 
the  periphery,  as  is  the  rule  in  insect  development. 
The  "  nucleolo"  remains  during  this  cleavage  period 
unchanged  near  the  posterior  end  (Fig.  44,  5) ;  then, 
when  cell  walls  appear,  it  becomes  distributed  among 

several  of  the  cells 
thus  formed.  These 
multiply  less  rapidly 
than  the  other  em- 
bryonic cells  and  are 
the  only  cells  that 
give  rise  to  the  germ 
cells  in  the  adult. 
It  is  thus  obvious 
that  there  is  here  an 
early  segregation  of 
germ  cells  and  that 
these  germ  cells  dif- 
fer from  the  somatic 
cells  in  the  posses- 
sion of  part  of  the 
{From  disintegrated  "nu- 
cleolo." 

The  polyembryonic  species  described  by  Silvestri 
are  Copidosoma  {Litomastix)  truncatellus  and  Agenias- 
pis  (Encyrtus)  fuscicollis.  The  eggs  of  these  species 
when  laid  are  vase-shaped  (Fig.  45),  the  posterior 
end  corresponding  to  the  base  of  the  vase.  Here 
also  a  germinal  vesicle  and  "nucleolo"  are  present, 
the  latter  almost  always  near  the  posterior  end. 
Parthenogenetic  eggs  were  found  to  produce  males. 


A  B  O 

Fig.  44.  —  Oophthora.     A.  Egg  with  germ- 
inal   vesicle    (A)    and    "Nucleolo"    (A''). 

B.  Egg  containing  many  cleavage  nuclei. 

C.  Formation   of  primordial    germ   cells 
(G)  at  posterior  end  of  an  egg. 
Silvestri,  1908.) 


Fig 


45  —Copidosoma  (Litomastix)  truncatdlu^.  A.  Oocyte  showing 
germinal  vesicle  {g.v)  containing  a  chromatin-nucleolus  (c.n)  and  a 
plasmosome  KP)-  B.  Egg  a  few  minutes  after  deposition  showing 
first  maturation  spindle  {m.s)  and  "  Nucleolo"  (A).  C.  Egg  ;ibout 
one  hour  after  deposition,  showing  three  polar  bodies  (p.6),  the 
first  cleavage  nucleus  and  the  "Nucleolo."  D.  Egg  m  two-cell 
stage,  about  one  and  one-half  hours  old.  p.n  =  Pohir  nucleus. 
E.  Four-cell  stage.  F.  Egg  about  four  and  one-half  hours  old 
showing  two  polar  nuclei  dividing,  two  embryonic  cells  containing 
nucleolar  substance,  and  six  embryonic  cells  (dividing)  ^ithout 
nucleolar  substance.     {From  Silvestri,  1906.)  (i-^'^ 


148        GERM-CELL  CYCLE   IN  ANIMALS 

whereas  fertilized  eggs  give  rise  to  females.  First 
and  second  polar  bodies  are  formed  and  the  first 
divides,  thus  making  three  in  all.  The  events  of 
early  cleavage  are  the  same  whether  the  nucleus 
consists  of  the  female  pronucleus  only  or  of  the 
female  and  male  pronuclei  fused.  Unlike  the  eggs 
of  monembryonic  species,  the  cleavage  nuclei  here  be- 
come separated  from  one  another  by  cell  walls  and 
the  "  nucleolo"  from  the  very  beginning  is  segregated 
at  each  division  in  a  single  cleavage  cell  (Fig.  45,  D). 
This  cell  divides  more  slowlv  than  the  others;  the 
"nucleolo"  gradually  becomes  vacuolated,  breaks 
down,  and  finally  is  evenly  scattered  throughout 
the  entire  cytoplasm.  Just  before  the  sixteen-cell 
stage  is  reached  the  cell  containing  the  disintegrated 
"nucleolo"  divides  and  the  two  daughter  cells  are 
provided  with  equal  amounts  of  its  substance  (Fig. 
45,  F).  Silvestri  was  only  able  to  trace  the  cells 
containing  the  remains  of  the  "nucleolo"  until 
four  of  these  were  present.  Nevertheless,  he  con- 
cludes that  these  and  these  alone  give  rise  to  the 
germ  cells.  This  conclusion  seems  well  founded  when 
the  history  of  this  "nucleolo"  is  compared  with 
that  of  similar  bodies  (keimbahn-determinants) 
in  the  eggs  of  certain  other  animals. 

Two  regions  develop  in  the  eggs  of  these  polyem- 
bryonic  Hymenoptera  :  (1)  an  anterior  or  polar  re- 
gion containing  the  polar  bodies,  and  (2)  the  posterior 
embryonic  region.  The  latter  again  becomes  differen- 
tiated into  two  regions  :  (1)  an  anterior  "massa  germi- 
nigera, "  which  gives  rise  to  normal  larvae,  and  (2)  a 


GERM   CELLS   IN   THE   ARTHROPOD  A     149 

posterior  "  massa  monem])rioiiale, "  which  produces 
the  so-called  asexual  larvtie.  These  lack  reproductive, 
respiratory,  circulatory,  and  excretory  systems.  They 
are  supposed  to  develop  from  cell  masses  which  do 
not  contain  descendants  of  the  cell  with  "  nucleolar" 
material,  and  to  serve  the  purpose  of  tearing  apart 
the  organs  of  the  host,  thus  making  it  available  as 
food  for  the  normal  larvae.  The  "  massa  monem- 
brionale,"  according  to  this  view,  consists  entirely  of 
somatic  cells,  whereas  the  "  massa  germinigera" 
possesses  both  somatic  and  germ  cells.  Doubts 
have  been  expressed  regarding  the  development  of 
the  asexual  larvae,  and  Silvestri's  results  need  con- 
firmation. There  seems  to  be  no  doubt  that  the 
"nucleolo"  is  a  keimbahn-determinant  in  both 
monembryonic  and  polyembryonic  Hymenoptera, 
but  its  identification  as  the  nucleolus  from  the  oocyte 
nucleus  did  not  seem  to  the  writer  to  be  well  estab- 
lished. Its  history  was,  therefore,  studied  by  the 
writer  (Hegner,  19146)  during  the  growth  period  of 
the  eggs,  with  the  following  results. 

My  material  consisted  of  a  brood  of  females 
belonging  to  the  polyembryonic  species  Copidosoma 
gelechicB.  As  in  most  other  insects,  the  two  ovaries  of 
Copidosoma  consist  of  rows  of  oocytes  in  various 
stages  of  growth  —  the  oldest  and  largest  near  the 
posterior  end,  and  the  youngest  and  smallest  at  the 
opposite  pole.  Before  the  oogonia  enter  the  growth 
period  (Fig.  46,  A,  o)  each  becomes  surrounded  by 
a  follicular  epithelium  (fe)  and  is  provided  with  a 
group  of  nurse  cells  (nc)  which  likewise  are  enclosed 


nc 


f  e 


Fig.  46.  —  Copidosoma  gelechice.  A.  Young  oocyte  (o)  surrounded  by 
an  epithelium  (f.e)  and  accompanied  by  nurse  cells  (n.c).  B.  Older 
oocyte  with  nurse  string  {n.s).  C.  Oocyte  containing  spindle  on 
which  are  pairs  of  chromosomes.  D-G.  Stages  in  condensation  of 
this  spindle.  //.  Cross  section  of  spindle  in  stage  shown  in  C. 
I.  Cross  section  of  spindle  in  stage  shown  in  D.  J-K.  Late  stages 
in  condensation  of  spindle.  (150) 


GERM   CELLS   IN  THE   ARTHROPOD  A    151 

by  a  cellular  envelope.  Increase  in  size  takes  place 
synchronously  in  both  the  nucleus  and  the  cytoplasm 
of  the  oocyte,  and  a  number  of  stages  in  this  process 
are  illustrated  in  the  accompanying  figures.  In 
Fig.  46,  B  a  strand  of  cytoplasm  is  shown  extending 
forward  to  the  nurse  chamber,  and  it  is  evidently  by 
means  of  this  pathway  that  nutritive  material  is 
conveyed  to  the  oocyte.  During  the  growth  period 
the  nurse  cells  decrease  in  size  until  they  occupy  but 
a  very  small  space  and  the  follicular  epithelium 
becomes  very  much  attenuated  (compare  Figs.  46,  A 
and  47,  D). 

The  fully  developed  oocytes  (Fig.  47,  D)  are  more 
or  less  vase-shaped  with  a  broad  base  (posterior), 
a  narrower  "  waist-line,"  and  a  slightly  thicker  distal 
(anterior)  portion.  They  are  not  so  long  and  slender 
as  those  illustrated  by  Silvestri,  but  perhaps  this 
shape  is  attained  later  when  the  eggs  are  laid. 
Within  the  oocyte  are  two  conspicuous  bodies.  At 
the  anterior  end  is  a  very  large  nucleus  {n)  which 
almost  completely  fills  that  portion  of  the  egg;  it 
contains  a  few  scattered  rods  of  chromatin.  Near 
the  posterior  end  is  a  smaller  but  even  more  con- 
spicuous body  (Fig.  47,  D,  k)  which  stains  very  deeply 
with  iron-haematoxylin.  This  may  be  vacuolated 
and  irregular,  showing  signs  of  disintegration,  as 
shown  in  Fig.  47,  or  may  possess  a  smooth  outline  and 
be  entirely  homogeneous.  It  is  undoubtedly  of  a 
very  tough  nature,  since  it  not  infrequently  tears  out 
of  the  egg  substance  when  struck  by  the  sectioning 
knife.     This  obviously  represents  the  "nucleolo"  of 


152        GERM-CELL  CYCLE  IN  ANIMALS 


Silvestri.  Silvestri  claims  that  this  "  nucleolo"  is  a 
plasmosome  which  was  cast  out  of  the  oocyte  nucleus 
at  an  early  stage  in  the  growth  period,  but  an  exami- 
nation of  my  material  proves  that  it  really  contains 
all  of  the  chromatin  of  the  oocyte  nucleus.     Since  it  is 


B 


Fig.  47.  —  Copidosoma  gelechice.  Stages  in  fusion  of  two  contiguous 
oocytes  end  to  end.  fe  =  epithelium  ;  k  =  keimbahn-chromatin  ; 
n  =  nucleus  ;  s  =  spindle  breaking  down  ;  u  =  point  of  union  of 
oocytes. 

not  a  nucleolus,  at  least  in  the  species  I  have  studied, 
it  can  no  longer  be  called  a  "  nucleolo"  and  therefore 
the  term  '  keimbahn-chromatin '  will  be  applied  to 
it. 

Figure  46,  A  was  drawn  from  a  longitudinal  section 
through  an  oocyte  (o)  in  an  early  stage  of  growth.  It 
is  surrounded  by  follicle  cells  (fe)  and  accompanied  by 


GERM   CELLS   IN   THE   ARTHROPOD  A     153 

a  group  of  nurse  cells  (n.c)  at  the  anterior  end.  A 
large  part  of  the  oocyte  is  occupied  by  the  nucleus 
(?i)  within  which  are  a  comparatively  few  irregular 
rods  of  chromatin,  forming  a  group  in  the  center. 
This  nucleus  thus  differs  quite  strikingly  from  those 
of  the  follicle  and  nurse  cells.  In  Fig.  46,  B  is 
shown  an  older  oocyte  and  two  of  the  accompanying 
nurse  cells  (n.c).  The  nucleus  contains  many  long 
slender  rods  of  chromatin  which  often  cross  each 
other  near  their  extremities. 

Soon  after  this  stage  is  reached  the  nuclear  mem- 
brane disappears  and  a  sort  of  spindle  is  formed  as 
illustrated  in  Fig.  46,  C.     No  asters  could  be  dis- 
covered,  but  the  spindle  fibers  are  quite  distinct. 
The  chromatin  rods  are  arranged  longitudinally  on 
the  spindle,  and  in  material  fixed  in  Carnoy's  solu- 
tion and  stained  in   iron-hsematoxylin  followed  by 
eosin,   are   remarkably   distinct.     The   arrangement 
of  these  rods  seems  to  indicate  either  that  entire 
chromosomes  are  separating  after  synapsis  or  that 
daughter     chromosomes     are     being     pulled     apart 
after   a    longitudinal    split.      I    am    unfortunately 
unable  to  state  definitely  what  processes  do  precede 
the  condition  shown  here,  but  it  seems  probable  that 
the  chromatin  of  the  early  oocytes  forms  a  spireme 
which  breaks  up  into  chromosomes,  and  that  these 
chromosomes  become  united  in  pairs  at  or  near  their 
ends,  and  are  there  drawn  out  upon  the  s]Mndle  as 
represented  in  Fig.  46,  C.     It  seems  also  certain  tliat 
a   definite   number   of   these   chromosome-pairs   are 
present.     Only  a  few  cross  sections  of  spindU-s  were 


154        GERM-CELL   CYCLE   IN  ANIMALS 

found  in  my  preparations,  but  in  these  the  chromo- 
somes are  widely  separated  and  consequently  easily 
counted.  Apparently  there  are  twelve  double  rods 
in  each  spindle  (Fig.  46,  H,  I), 

Instead  of  continuing  its  activity  and  forming  two 
daughter  nuclei  this  spindle  persists  for  a  long  time, 
undergoing  a  gradual  contraction  and  condensation. 
Thus  in  the  stage  succeeding  that  just  described  the 
chromatin  rods  are  close  together  and  the  entire 
spindle  has  decreased  in  diameter  although  not  in 
length  (Fig.  46,  D) .     Spindles  in  this  condition  are  not 
always  parallel  to  the  long  axis  of  the  egg  but  may  be 
oblique  or,  more  rarely,  almost  perpendicular  to  this 
axis.     Hence  several   transverse  sections   were  ob- 
tained, one  of   which  is  illustrated  in   Fig.   46,   7. 
Here  also  is  shown  a  closer  proximity  of  the  chromo- 
somes as  compared  with  the  cross  section  of  the 
younger   spindle  represented   in   Fig.    46,   H.     The 
number  of  chromosomes  also  appears  to  be  constant, 
namely,     twelve.     During     succeeding     stages    the 
spindle  continues  to  shorten  and  condense.     That 
shown  in  Fig.  46,  E  still  exhibits  spaces  between 
the  rods  and  the  presence  of  only  a  few  spindle  fibers. 
A  further  contraction  is  indicated  in  Fig.   46,   F, 
where   the    chromosomes    have   become   so    closely 
crowded   as  to  form   an   apparently   sohd  body  in 
the  shape  of   a  cross.     This   chromatin   body   still 
continues  to  contract  as  shown  in  Fig.  46,  G,  J,  and  K. 
At  about  this  time  vacuoles  begin  to  appear  within  it 
(Fig.  46,  K)  and  its  shape  becomes  more  or  less  irreg- 
ular, most  often  assuming  a  nearly  spherical  condi- 


GERM  CELLS  IN  THE  ARTHROPOD  A     155 

tion.  This  may  now  be  recognized  as  the  "  nucleolo  " 
of  Silvestri  or  the  keimbahn-chromatin  as  we  have 
decided  to  call  it. 

The  spindle  at  first  lies  nearer  the  anterior  than 
the  posterior  part  of  the  oocyte.  As  it  shortens 
and  condenses  it  is  more  often  found  below  the  middle 
of  the  cell,  and  finally  reaches  a  position  near  the 
posterior  end.  The  conclusion  is  thus  reached  that 
the  *' nucleolo"  of  Silvestri  is  not  a  plasmosome 
(metanucleolus)  which  escapes  from  the  oocyte 
nucleus,  but  consists  of  all  of  the  chromatin  of  this 
nucleus  condensed  into  a  more  or  less  spherical  body 
during  a  peculiar  process  of  spindle  formation. 

The  discovery  of  the  origin  and  nature  of  the  keim- 
bahn-chromatin   brought    forth    a    new    problem, 
namely,  that  of  the  origin  of  the  egg  nucleus.     It 
was  early  noted  that  the  oocytes  containing  this 
peculiar  spindle  were  free  from  any  other  inclusions 
in   the   cytoplasm.     How   then   do   they   acquire   a 
nucleus?      Two  hypotheses   have  been  considered, 
one  of  which  has  a  considerable  body  of  evidence  in 
its  support.     In  the  first  place  the  nucleus  might 
arise  from  chromatin  granules  which  break  away  from 
the  chromosomes  during  the  formation  or  conden- 
sation of  the  spindle.     There  is,  however,  no  evidence 
for  this  view,  since  the  entire  chromatin  content  of 
the  oocyte  nucleus  seems  to  take  part  in  the  forma- 
tion of  the  spindle  and  later  the  keimbahn-chromatin. 
The  second  hypothesis  was  suggested  when  a  number 
of  cases  were  discovered  of  two  oocytes  lying  end 
to  end  without  any  intervening  follicular  epithelium. 


156        GERM-CELL   CYCLE   IN  ANIMALS 

This  hypothesis  is  that  pairs  of  oocytes  unite  end  to 
end,  the  posterior  oocyte  containing  the  keimbahn- 
chromatin  and  the  anterior  furnishing  the  egg 
nucleus.  Stages  in  this  process  are  shown  in  Fig.  47, 
A,  B,C,  and  D. 

As  the  oocytes  increase  in  size  and  age  the  f olHcular 
epithehum  becomes  gradually  thinner  and  in  several 
instances  only  a  delicate  strand  could  be  observed 
between  the  ends  of  adjoining  oocytes.  In  Fig.  47,  A 
two  oocytes  are  shown  without  any  cellular  layer 
between  them,  although  the  follicular  epithelium 
extends  in  a  short  distance  at  the  point  of  contact. 
The  posterior  cell  is  much  the  larger  and  older,  and 
possesses  keimbahn-chromatin,  but  no  nucleus. 
The  other  oocyte  is  younger  and  smaller  and  con- 
tains what  has  been  interpreted  as  a  disintegrating 
spindle  (s).  The  condition  illustrated  in  Fig.  47,  B 
is  similar  except  that  the  keimbahn-chromatin  in 
the  posterior  oocyte  is  less  regular,  having  already 
begun  to  break  up,  and  the  chromatin  rods  in  the 
anterior  cell  represent  a  further  stage  in  the  trans- 
formation of  a  spindle  into  a  nucleus.  Figure  47,  C 
illustrates  what  is  considered  a  later  stage  in  the  fusion 
process.  The  anterior  part,  which  contains  a  definite 
nucleus,  is  connected  with  the  posterior  position 
by  a  thick  strand.  The  nuclear  membrane  is  not 
very  distinct  in  the  preparation  indicating  that  the 
nucleus  is  not  yet  completely  formed.  The  posterior 
part  is  not  as  large  as  in  the  other  figures,  since  the 
section  was  not  exactly  in  the  longitudinal  axis,  but 
slightly  oblique.     The  keimbahn-chromatin  has  been 


GERM   CELLS   IN   THE   ARTHR0P(3DA     lo7 

added  in  the  figure  from  a  part  of  the  oocyte  three 
sections  away.  A  still  further  stage  of  fusion  is 
indicated  in  Fig.  47,  D. 

In  all  these  cases  and  in  fully  developed  eggs 
there  is  a  distinct  "waist  line"  which  can  be  ac- 
counted for  upon  the  view  that  two  oocytes  fuse  end 
to  end  as  above  described,  the  narrow  part  corre- 
sponding to  the  region  of  union.  The  conclusion 
seems  warranted,  therefore,  that  every  egg  when  laid 
consists  of  two  oocytes  which  have  united  end  to  end, 
the  posterior  or  older  oocyte  being  provided  with 
keimbahn-chromatin  derived  from  the  chromatin  of 
its  nucleus,  and  the  anterior  supplied  with  a  nucleus 
which  has  arisen  from  the  disintegration  of  a  spindle 
similar  to  that  from  which  the  keimbahn-chromatin 

originated. 

A  number  of  references  are  present  in  literature  to 
what  have  been  termed  "uterine,"  "disappearing," 
or  "aborting"  spindles.  Such  a  spindle  was  first 
noted  by  Selenka  (1881)  in  the  turbellarian,  Tlujsano- 
zoori  diesingii.  Here  apparently  a  completely  de- 
veloped maturation  spindle  was  observed  in  the 
fully  grown  eggs  after  they  had  entered  the  uterus; 
then,  just  before  the  metaphase  of  mitosis,  the  spindle 
broke  down  and  the  nucleus  returned  to  a  resting 
condition.  This  same  nucleus  later  gave  rise  to 
polar  bodies  as  in  the  eggs  of  other  animals.  Similar 
aborting  spindles  have  been  described  by  Lang  (1884) 
in  several  species  of  polyclads,  by  Wheeler  (1894)  in 
Planocera  inquilina,  by  Gardiner  (1895,  1898)  in 
Polychoerus  caudatus,  by  Surface  (1907)  in  Planocera, 


158        GERM-CELL   CYCLE   IN  ANIMALS 

by  Patterson  (1912)  in  Graffilla  gemellipara,  and  by 
Patterson  and  Wieman  (1912)  in  Planocera  inquilina. 
Patterson  and  Wieman  have  given  the  uterine  spindle 
in  Planocera  careful  study,  and  have  established  the 
fact  that  in  this  species  it  is  simply  a  maturation 
spindle  which  forms  near  the  center  of  the  egg  and 
later  moves  to  the  periphery,  undergoing  during 
this  migration  a  distinct  contraction.  They  further 
suggest  that  the  uterine  spindles  described  in  the 
eggs  of  other  animals  are  really  one  phase  in  a  typical 
maturation  process. 

It  has  thus  been  shown  that  the  first  maturation 
spindle  in  certain  eggs  may  remain  practically  in- 
active for  a  considerable  period.  It  should  be  noted, 
however,  that  in  Copidosoma  the  spindle  arises  not 
in  the  fully  grown  egg  but  in  very  young  oocytes, 
and  that  it  appears  to  lack  asters  at  every  period  of 
its  history.  While  therefore  this  structure  may  be  a 
precocious  maturation  spindle,  it  differs  markedly 
from  any  other  such  spindle  that  I  have  been  able 
to  find  described  in  cytological  literature. 

The  second  view  is  that  the  oocyte  spindle  repre- 
sents a  special  mechanism  leading  to  an  accurate 
distribution  of  chromatin  in  the  keimbahn-chromatin 
mass.  The  position  of  the  contracted  and  condensed 
spindle,  however,  is  not  definite,  since  it  has  been 
found  to  occupy  almost  any  part  of  the  oocyte  and 
to  lie  with  its  long  axis  parallel  to  the  long  axis  of 
the  oocyte,  or  oblique  or  even  perpendicular  to  this 
axis  (Fig.  46,  E,  G).  Furthermore  the  keimbahn- 
chromatin  does  not  seem  to  be  of  a  definite  structure. 


GERM   CELLS   IN   THE   ARTHROPOD  A     159 

but  soon  after  it  reaches  a  sphere-like  shape  it  begins 
to  vacuolate  and  becomes  irregular  (Figs.  46,  K ;  47). 
It  also  seems  probable  that  in  some  oocytes  the 
oocyte  spindle  gives  rise  to  the  keimbahn-chromatin, 
whereas  in  others  it  becomes  disorganized,  forming 
the  nucleus  of  the  egg  (Fig.  47,  A,  B,  C).  What 
causes  the  difference  in  the  history  of  the  oocyte 
spindles  ?  No  definite  answer  can  be  given  to  this 
question,  but  there  are  two  possibilities,  (1)  external 
and  (2)  internal  influences.  It  seems  very  improb- 
able that  any  internal  mechanism  exists  which 
determines  what  the  history  of  the  oocyte  spindle 
shall  be.  On  the  other  hand,  the  arrangement  of 
the  oocytes  in  the  ovary  might  cause  the  spindle  of 
those  most  posteriorly  situated  to  become  keimbahn- 
chromatin  and  of  those  next  in  order  to  transform 
into  nuclei.  According  to  this  view  the  oocytes  de- 
pend upon  chance  for  their  final  position  in  the 
ovary,  and  the  fate  of  the  spindle  is  decided  by  the 
environment  of  the  oocyte. 

There  are  numerous  cases  of  cell  fusion  in  both 
Protozoa  and  Metazoa,  and  germ  cells  and  somatic 
cells.  For  example.  Protozoa  engulf  other  cells ; 
the  fully  grown  ova  of  Hydra  consist  of  several 
germ  cells  fused  together ;  and  leucocytes  may  fuse 
with  one  another.  In  all  such  cases  the  nucleus  of 
one  cell  persists,  whereas  those  of  the  other  cells 
disintegrate  and  disappear.  Among  certain  leuco- 
cytes of  Axolotl,  however.  Walker  (1907)  has  de- 
scribed a  sort  of  fusion  which  results  in  the  trans- 
ference of  the  chromatin  from  one  cell  to  another 


160         GERM-CELL   CYCLE   IN  ANIMALS 

without  the  disintegration  of  the  migrating  chroma- 
tin. In  plants  also  Gates  (1911)  has  shown  that 
chromatin  may  migrate  from  one  pollen  mother-cell 
of  (Enothera  gigcis  into  a  neighboring  mother-cell 
where  it  remains  visible  for  some  time  before  be- 
coming incorporated  with  the  surrounding  cyto- 
plasm. Many  more  cases  of  cellular  fusion  might  be 
mentioned,  but  in  no  instance  so  far  as  I  am  aware 
has  the  union  of  two  well-developed  oocytes  to  form 
one  egg  been  reported.  It  is  true  that  in  Copido- 
soma  the  chromatin  in  one  (the  proximal)  oocyte 
(the  keimbahn-chromatin)  finally  disintegrates  and 
disappears  in  the  cytoplasm,  and  thus  the  condition 
here  may  be  compared  with  that  in  the  cases  men- 
tioned above,  but  the  stage  of  fusion  in  Copidosoma 
is  extremely  late  in  the  growth  period,  and  the 
chromatin  material  remains  visible  for  a  remarkably 
long  interval  of  the  germ-cell  cycle. 

According  to  Silvestri  the  first  cleavage  cell  of 
Copidosoma  consists  of  the  egg  nucleus  surrounded 
by  only  a  small  portion  of  the  substance  in  the  pos- 
terior end  of  the  egg  in  which  is  embedded  the  keim- 
bahn-chromatin. If  the  two  materials  within  the 
oocytes  do  not  become  intimately  fused,  it  is  obvious 
therefore  that  the  cells  of  the  embryo  which  are 
descended  from  the  first  cleavage  cell  are  derived 
from  the  nucleus  of  the  anterior  of  the  two  fused 
oocytes  and  cytoplasm  from  the  posterior  oocyte 
with  the  addition  of  the  keimbahn-chromatin. 

The  history  of  the  germ  cells  after  their  segrega- 
tion is  not  known  for  any  polyembryonic  animal. 


GERM   CELLS   IN  THE  ARTHROPOD  A     161 

Polyembryony  has  been  described  in  an  earthworm. 
Lumbricus  trapezoides  (Kleinenberg,  1879),  in  cer- 
tain Bryozoa  (Harmer,  1893;  Robertson,  1903), 
in  the  armadillo  (Patterson,  1913),  and  in  parasitic 
Hymenoptera  (Marchal,  1904;  Silvestri,  1906, 
1908) .  In  every  case  cleavage  is  of  the  indeterminate 
type,  and  the  cell  lineage  is  unknown.  Various 
theories  have  been  advanced  to  account  for  poly- 
embryony, such  as  (1)  blastotomy  or  the  early 
separation  of  blastomeres,  each  giving  rise  to  a 
single  individual  as  has  been  brought  about  by 
Driesch  (1892)  and  others  by  separating  the  blas- 
tomeres of  the  eggs  of  certain  animals ;  (2)  polyovular 
follicles  may  occur  in  mammals  and  by  some  (Rosner, 
1901)  are  considered  sufficient  to  account  for  poly- 
embryony among  the  members  of  this  class ;  and 
(3)  precocious  budding  has  been  suggested  to  account 
for  the  production  of  man}^  individuals  from  a 
single  egg,  most  recently  by  Patterson  (1913), 
who  has  shown  that  in  the  armadillo  the  blastoderm 
produces  two  primary  buds  from  each  of  which  two 
secondary  buds  arise,  and  hence  four  young  develop 
from  each  egg.  According  to  the  theory  of  germinal 
continuity  each  of  the  buds  must  be  supplied  with 
germ  cells  or  with  germ-plasm  which  has  not  yet 
been  segregated  into  germ  cells.  Silvestri \s  in- 
vestigations seem  to  indicate  that  the  former  is 
true  for  parasitic  Hymenoptera,  but  it  is  difficult 
to  see  how  a  definite  number  of  germ  cells  can  be 
supplied  to  each  bud  during  a  process  of  development 
which  is  apparently  so  indeterminate.     If,  however,  a 

M 


16^        GERM-CELL  CYCLE  IN  ANIMALS 

definite  number  is  not  required,  and  the  germ  cells 
become  generally  distributed  throughout  the  cellular 
mass  before  budding  begins,  the  chances  are  that 
every  bud  will  contain  one  or  more  germ  cells.  For 
example,  if  germ  cells  occur  in  all  parts  of  the  blas- 
toderm of  the  armadillo,  as  is  quite  possible,  each  of 
the  four  embryos  must  become  provided  with  a  por- 
tion of  them.  On  the  other  hand,  the  germ-plasm 
may  be  rather  widely  distributed  among  the  cells 
and  only  becomes  segregated  in  germ  cells  after  bud- 
ding takes  place.  Careful  studies  of  the  germ-cell 
history  in  polyembryonic  species  are  much  needed 
and  would  no  doubt  produce  important  results. 

The  data  presented  in  this  chapter  are  sufficient 
to  prove  that  in  many  insects  a  complete  germ-cell 
cycle  can  be  demonstrated.  There  are  many  species, 
however,  in  which  no  early  segregation  of  germ  cells 
has  been  discovered  even  after  very  careful  examina- 
tion. It  is  therefore  too  early  to  make  any  general 
statements  for  the  entire  class,  but  we  must  base  our 
conclusions  regarding  the  germ-cell  cycle  upon  our 
knowledge  of  those  forms  in  which  the  keimbahn 
actually  can  be  traced.  Finally  one  point  should  be 
emphasized ;  in  every  case  the  segregation  of  the 
primordial  germ  cells  is  intimately  associated  with  a 
substance  which  can  be  made  visible  by  proper 
staining  methods.  In  Miastor  this  is  the  pole- 
plasm;  in  Chironomus  the  "Keimwulst"  or  **Keim- 
bahnplasma"  ;  in  Calliphora  the  "  Dotterplatte " ; 
in  chrysomelid  beetles  the  pole-disc ;  and  in  parasitic 
Hymenoptera,  the  keimbahn-chromatin.     The  na- 


GERM   CELLS   IN   THE   ARTHROPOD  A     163 

ture  and  significance  of  these  substances  will  be  dis- 
cussed later. 

2.  The  Keimbahn  in  the  Crustacea 

The  keimbahn  in  the  Crustacea  is  best  known 
in  certain  Cladocera  and  Copepoda.  Of  special 
interest  are  the  investigations  of  Grobben  (1879), 
Weismann  and  Ischikawa  (1889),  Haecker  (1897), 
Amma  (1911),  Kuhn  (1911,  1913),  and  Fuchs  (1913). 

Grobben  (1879)  studied  the  embryology  of  Moina 
rectirostris  and  gives  a  remarkably  fine  account  of 
early  cleavage  stages,  considering  the  early  date 
when  the  work  was  done.  He  figures  stages  showing 
a  foreign  body  which  he  considered  a  polar  body, 
segregated  in  one  of  the  early  blastomeres,  the  segre- 
gation and  characteristics  of  the  primordial  germ 
cell  and  the  first  entoderm  cell,  and  the  division  and 
later  history  of  the  germ  cells.  His  results  have  been, 
in  the  main,  confirmed  by  Ktihn  (1911,  1913). 

Weismann  and  Ischikawa  (1889)  have  contributed 
an  interesting  account  of  the  primary  cellular  differ- 
entiation in  the  fertilized  winter  eggs  of  six  species 
of  the  Daphnid^,  belonging  to  four  genera.  The 
germinal  vesicle  in  the  eggs  of  these  species  casts 
part  of  its  chromatin  contents  into  the  cytoplasm 
which  there  became  organized  into  a  "Paranucleus." 
This  paranucleus  then  acquired  a  cell  body  and  in 
this  condition  was  termed  the  "Copulationszelle" 
because  of  its  future  history.  In  two  of  the  species 
examined  this  Copulationszelle  united  with  one  of 
the  first  two  cleavage  cells ;  in  the  other  four  species 


164        GERM-CELL   CYCLE   IN   ANLVLVLS 

it  united  with  one  of  the  first  eight  cells.  Further- 
more, it  apparently  always  fused  with  a  certain 
definite  cleavage  cell.  The  authors  conclude  that  the 
Copulationszelle  has  some  important  relation  to  the 
history  of  the  germ  cells. 

The  keimbahn  of  Cyclops  and  some  closely  allied 
forms  has  been  very  carefully  investigated  by  Haecker 


ak-' 


Fig.  48.  —  Cyclops.  A.  Egg  showing  "  Aussenkornchen "  (ak)  at  one 
end  of  first  cleavage  spindle.  B.  Thirty-two-cell  stage  showing 
"  Aussenkornchen "  (ak)  in  the  primordial  germ  cell  (Kz).  Rk  = 
polar  bodies.     (From  Haecker,  1897.) 

(1897),  Amma  (1911),  and  Fuchs  (1913)  with  results 
which  are  of  particular  interest.  In  Cyclops,  accord- 
ing to  Haecker,  "Aussenkornchen"  arise  at  one  pole 
of  the  first  cleavage  spindle  (Fig.  48,  A,  ak) ;  these 
are  derived  from  disintegrated  nucleolar  material 
and  are  attracted  to  one  pole  of  the  spindle  by  a  dis- 
similar influence  of  the  centrosomes.  During  the 
first  four  cleavage  divisions  the  granules  are  segregated 
always  in  one  cell  (Fig.  48,  B,  Kz) ;  at  the  end  of  the 
fourth  division  these  "Aussenkornchen"  disappear, 
but  the  cell  which  contained  them  can  be  traced  by 
its  delayed  mitotic  phase  and  is  shown  to  be  the 
primordial  germ  cell. 


GERM   CELLS   IN   THE   ARTHROPODA     165 

The  most  recent  and  complete  accounts  of  the 
keimbahn  in  the  Copepoda  are  those  of  Amma  (1911) 
and  Fuchs  (1913).  Amma  studied  the  early  cleavage 
stages  of  eleven  species  of  Cyclops  (Fig.  49,  A-G), 
three  species  of  Diaptomus  (Fig.  49,  //),  one  species 
of  Canthocamptus,  and  one  species  of  Ileterocope. 
Cyclops  fucus  var.  distinctus  is  made  the  basis  for 
the  most  detailed  study,  but  short  descriptions  and 
figures  are  presented  of  the  others.  In  all  of  the 
sixteen  species  examined  the  stem-cell  which  gives 
rise  to  the  primordial  germ  cell  may  be  recognized, 
as  Haecker  (1897)  discovered  in  Cyclops,  first  by  the 
presence  of  granules  which  do  not  occur  in  the  other 
cleavage  cells,  and  later  by  a  delayed  mitotic  divi- 
sion. The  process  is  essentially  as  described  by 
Haecker.^ 

^  The  following  summary  of  the  keimbahn  in  Cyclops  fuscus  var.  dis' 
tindus  is  given  by  Amma : 

"1.  Wahrend  der  ersten  Furchungsteilungen  ist  eine  bestimmte 
Folge  von  Zellen,  die  Keimbahn,  durch  das  Auftreten  von  Kornchen, 
die  sich  bei  der  Teilung  jewels  um  einen  Spindelpol  der  Teilungsfigur 
ansammeln   gekennzeichnet  (Fig.  49,  A). 

"2.  Die  Kornchen  oder  Ectosomen  entstehen  immer  erstmals  wahrend 
des  Stadiums  der  Diakinese,  vermehren  sich  wahrend  der  niichst  folgen- 
den  Phasen  noch  bedeutend  und  verschmelzen  gegen  das  Ende  der 
Teilung  zu  grosseren,  unformigen  Brocken,  welche  allmahlich  ^^■iih^cnd 
des  Ruheperiode  der  Zelle  aufgelost  werden  (Fig.  49,  B). 

"3.  Die  neue  Kornchenzelle  geht  stets  vom  kornchenfiihronden 
Produkte  der  alten  Kornchenzelle  hervor,  was  dirckt  dadurch  hcwiesen 
werden  kann,  dass  sich  in  der  neuen  Kornchenzelle  immer  noch  unauf- 
geloste  Uberreste  der  Ectosomen  der  alten  Kornchenzelle  vorHndon  ; 
alle  Komchenzellen  stammen  somit  in  direkter  Linie  von  einander  ab 
(Fig.  49,  C). 

"4.  Vom  II  —  Zellenstadium  an  bleibt  die  Kornchenzelle  imincr  in 
der  Teilung  hinter  den  andem  Furchungszellen  zuriick ;    es  ergibt  sich 


z    u     E^ 


Fig.  49. 


Stages  in  the  keimbahn  of  copepods.  A-G.  Cyclops  fuscus 
var.  distinctus.  H.  Diaptomus  coeruleiis.  I.  Cyclops  viridis. 
A.  Ectosomes  at  end  of  first  cleavage  spindle.  B.  Two-cell  stage; 
ectosomes  dissolving.  C.  Old  and  newly  formed  ectosomes  at  end 
of  one  of  second  cleavage  spindles.  D.  Eight-cell  stage ;  ectosomes 
dissolving  in  stem-cell.  E.  Sixteen-  to  twenty-eight-cell  stage. 
S  =  cell  with,  E  =  cell  without,  granules.  F.  One  hundred  and 
twelve-cell  stage  with  two  primordial  germ  cells  (u)  and  three  en- 
toderm cells  (E).  G.  Two  hundred  and  forty-cell  stage,  u  =  pri- 
mordial germ  cells.  H.  Appearance  of  ectosomes  before  cleavage 
spindle  forms.  /.  Increased  production  of  ectosomes  due  to  car- 
bonic acid  gas.     (From  Amma,  1911.)  (166) 


GERM   CELLS  IN  THE  ARTHROPOD  A     1G7 

An  important  departure  from  the  usual  method 
of  origin  of  the  "Ectosomen"  is  recorded  for  Diapto- 
mus  cceruleus.  Amma  says  concerning  the  process 
in  this  species  that  "  whereas  in  other  forms  the  Ecto- 
somen  first  appear  during  the  stage  of  diakinesis  of 
the  first  cleavage  spindle,  in  this  species  they  are 
already  present  before  the  pronuclei  unite"  (Fig. 
49,  H). 

The  origin  and  nature  of  the  Ectosomen  are  con- 
sidered by  Amma  at  some  length.  The  hypothesis 
that  these  granules  arise  by  the  splitting  off  of  parti- 
cles of  chromatin  from  the  chromosomes  as  occurs  in 
Ascaris  is  rejected  (1)  because  in  one  species,  Diap- 
tomus  coeruleus  (Fig.  49,  H),  the  Ectosomen  appear 
before  the  nuclear  membrane  breaks  down  in  prepara- 
tion for  the  formation  of  the  first  cleavage  spindle, 
and  (2)  because  the  Ectosomen  do  not  stain  as  deeply 
as  chromatin  but  only  slightly  darker  than  the  cyto- 
plasm. The  origin  of  the  Aussenkornchen  (Ectoso- 
men) from  the  nucleolus,  as  considered  probable  by 

eine  Phasendifferenz,  welche  in  immer  starkeren  Masse  in  den  hoheren 
Furchungsteilungen  zunimmt  (Fig.  49,  C). 

"5.  Aus  dem  kornchenfuhrenden  Produkte  der  Kornchenzelle  des 
vierten  Teilungsakts,  der  Stammzelle  S,  gehen,  nachdem  diese  sich  an 
dem  fiinften  Furchungsschritte  nicht  beteiligte,  gegen  Ende  des  sechsten, 
im  LX  —  Zellenstadium,  die  beiden  definitiven  Urgeschlechtszellen  her- 
vor ;  bei  dieser  Teilung  der  S-Ze\\c  erscheinen  die  Ectosomen  in  ganzen 
Zellraume  (Fig.  49,  E,  F). 

"6.  In  Ausnahmefiillen  beginnt  die  S-Zelle  sich  etwas  friiher  zu  teilen, 
namlich  schon  wahrend  des  Ubergangs  des  XXX  —  zum  LX  —  Zellen- 
stadium. 

"7.  Die  Urgeschlechtszellen  verlieren  den  Verband  mit  dem  Blasto- 
derm, sie  werden  allmahlich  in  die  Tiefe  gedriingt  (Fig.  49,  G)" 
(pp.  529-530). 


168        GERM-CELL   CYCLE   IN  ANIMALS 

Haecker  (1897),  could  not  be  confirmed.  The  con- 
dition in  Diaptomiis  coerideus  (Fig.  49,  H)  is  also  a 
serious  objection  to  this  theory.  The  Ectosomen  are 
different  from  chromidia,  since  chromidia  arise  from 
the  nucleus  and  no  connection  could  be  discovered 
between  the  Ectosomen  and  the  nuclei.  The  hy- 
pothesis that  they  may  represent  chondriosomes  is 
also  rejected. 

Amma  finally  decides  ^  that  the  Ectosomen  repre- 
sent the ' '  Endprodukte  des  Kern-Zelle-Stoff  wechsels, ' ' 
in  which  case  a  greater  amount  of  Ectosomen  would 
be  present  if  an  egg  were  allowed  to  develop  in  car- 
bonic acid  gas.     The  results  of  a  number  of  experi- 

^  "  Aus  dem  ganzen  Verlaufe  der  K6rnchenent^\acklung  geht  nun  so\ael 
mit  Sicherheit  hervor,  dass  man  es  bei  den  Ectosomen  mit  vergang- 
lichen  Gebilden  zu  tun  hat,  denen  keine  weiteren  Funktionen  zukommen, 
die  im  Leben  der  Zelle  nicht  weiter  verwendet  werden.  In  den  Prophasen 
der  Kernteilung  entstehen  die  Kornchen  zunachst  als  feine  Tropfchen  im 
Zellplasma ;  im  weiteren  Verlauf  der  Teilung  erfahren  sie  dann  noch 
eine  Zunahme,  bis  sie  ungefahr  im  Stadium  des  Dyasters  ihre  hochste 
Entwicklung  erreicht  haben.  Von  hier  ab  beginnt  der  regressive  Prozess 
der  Kornchen  :  sie  fliessen  zu  grosseren,  unformigen  Klumpen  zusammen, 
welche  vom  Zellplasma  allmahlich  voUstandig  resorbiert  und  aufgelost 
werden.  Bei  der  nachsten  Teilung  der  Keimbahnzelle  erscheinen  dann 
die  Ectosomen  wieder  von  neuem.  Um  ein  einfaches  Unsichtborwerden 
wahrend  der  Zellenruhe,  wie  es.  z.  B.  vom  Centrosoma  von  vielen 
Forschern  angenommen  wird,  kann  es  sich  bei  den  Ectosomen  nicht 
handeln,  denn  vielfach  konnten  ja  neben  den  neuen,  frisch  entstandenen 
Ectosomen  noch  die  Uberreste  der  Ectosomen  der  letzen  Kornchenzelle 
nachgewiesen  werden.  Es  erfolgt  also  bei  jedem  neuen  Teilungsschritte 
tatsachlich  eine  Neuhildung  und  Wiederauflosung  der  Kornchen. 

"  Gestutzt  auf  diese  Tatsachen,  mochte  ich  nun  die  Ansicht  vertreten, 
dass  die  Ectosomen  als  Ahscheidiingen,  Endprodukte  des  Kern-Zelle 
Stoffwechsels  aufzufassen  sind,  welche  zu  bestimmten  Zeiten  im  Plasma 
der  Zelle  zur  Abscheidung  gelangen  und  wieder  aufgelost  werden" 
(p.  557). 


GERM   CELLS   IN   THE   ARTHROPODA     169 

ments  with  oxygen  and  carbonic  acid  gas  indicate 
that  a  greater  amount  of  Ectosomen  occur  when  the 
egg  is  developed  in  the  latter,  as  shown  by  Fig.  49,  I, 
which  is  from  an  egg  of  Cyclops  viridis  placed  one 
hour  after  deposition  into  carbonic  acid  gas  for  one 
hour. 

When  various  stains  were  used  it  was  found  that 
the  Ectosomen  became  colored  much  like  the 
cytoplasm.  For  example,  when  stained  in  methylene 
blue  followed  by  eosin  the  chromosomes  were  blue 
and  the  Ectosomen  and  cytoplasm  red,  and  when 
stained  by  the  methyl  green-f  uchsin-orange  G  method 
of  Heidenhain  the  chromosomes  were  green  and 
the  cytoplasm  and  Ectosomen  red. 

Amma  also  attempts  to  explain  the  fact  that  the 
Ectosomen  appear  at  only  one  end  of  the  first  cleav- 
age spindle  and  in  only  one  of  the  cleavage  cells 
until  the  two  primordial  germ  cells  are  formed. 
He  rejects  the  hypothesis  Haecker  advanced  that  the 
centrosomes  possess  an  unequal  influence  upon  the 
Ectosomen  and  that  one  centrosome  attracts  all  of 
them  because  it  is  stronger  than  the  other,  and  is 
inclined  to  favor  the  idea  that  the  Ectosomen  are  the 
visible  evidence  of  an  organ-forming  substance  which 
is  thus  distinguished  from  the  rest  of  the  cytoplasm 
as  *'Kornchenplasma."  ^ 

Fuchs    (1913)    has   confirmed   for   Cyclops   viridis 

^  Amma*s  statement  is,  "dass  im  Zellplasma  des  noch  ungcfurchten 
Copepodeneies  ein  vom  iihrigen  Eiplasma  qualitativ  versclnedenes  Kbrn- 
chenplasma  existiert,  urlches  die  organbildende  Substanz,  die  Anlagesub- 
stanz  fur  die  Geschlechtsorgane  darstellt"  (p.  5G4). 


170        GERM-CELL   CYCLE   IN  ANIMALS 

many  of  Amma's  results  and  has  pointed  out  the 
similarities  between  the  cell  lineage  of  the  Copepoda 
and  Cladocera.  Kuhn  (1913)  has  studied  the  keim- 
bahn  in  the  summer  egg  of  a  cladoceron,  Polyphemus 
pediculus,  and  has  confirmed  certain  parts  and  cor- 


FiG.  50.  —  Polyphemus  pediculus.  A.  Egg  with  three  nurse  cells. 
B.  Egg  at  close  of  maturation,  n  =  "  Nahrzellenkern."  C.  Two- 
cell  stage ;  view  of  vegetative  pole.  D.  Eight-  to  sixteen-cell  stage. 
K  =''  keimbahnzelle."  E.  Sixteen-  to  thirty-cell  stage,  e  =  en- 
toderm cell.  F.  Thirty-two-cell  stage  from  vegetative  pole.  K  = 
primordial  germ  cells ;  e  =  entoderm  cells.     (From  Kiihn,  1911,1913.) 

rected  other  portions  of  the  work  done  by  earlier 
investigators  —  Grobben  (1879),  Samassa  (1893), 
and  Weismann  and  Ischikawa  (1889).  In  this 
species  usually  one  (but  sometimes  two  or  three)  of 
the  nurse  cells  (Fig.  50)  pass  into  the  egg  before 
cleavage.  This  cell  (or  cells)  becomes  embedded 
near  the  periphery  at  the  vegetative  pole  (Fig.  50, 
B,  n).     During  each  of  the  early  cleavage  divisions 


GERM   CELLS  IN  THE  ARTHROPODA     171 

this  nurse  cell  is  confined  to  one  cell  (Fig.  50,  C-E) 
which  gives  rise  during  the  third  cleavage  (8-  to  IG- 
cell  stage)  to  the  primordial  germ  cell,  containing 
the  remains  of  the  nurse  cell  (Fig.  50,  E,  K),  and  to 
the  primordial  entoderm  cell  which  does  not  receive 
any  part  of  the  nurse  cell  (Fig.  50,  E,  e).  The  pri- 
mordial germ  cell  and  primordial  entoderm  cell  do  not 
divide  as  quickly  as  the  other  blastomeres  during 
the  succeeding  cleavage  stages  —  a  fact  that  aids 
in  their  identification.  While  the  egg  is  undergoing 
cleavage  the  nurse  cell  is  gradually  changing,  so  that 
when  the  sixteen-cell  stage  is  reached  it  has  become 
disintegrated  into  dark  staining  granules  and  frag- 
ments of  various  forms  and  sizes  (Fig.  50,  E).  Dur- 
ing the  division  of  the  ''Keimbahnzelle"  (from  16- 
32-cell  stage)  these  granules  and  fragments  are  about 
equally  distributed  between  the  daughter  cells  (Fig. 
50,  F).  A  similar  distribution  takes  place  in  suc- 
ceeding divisions  of  the  primordial  germ  cells,  and 
this  is  accompanied  by  a  further  decrease  in  the  size 
of  the  dark  staining  granules.  A  blast ula  of  "^SG 
cells  is  figured  by  Kuhn  which  shows  at  the  vegeta- 
tive pole  four  primordial  germ  cells  lying  next  to 
eight  entoderm  cells  and  bordered  by  twelve  meso- 
derm cells.  During  gastrulation  this  grouj)  of  twenty- 
four  cells  becomes  surrounded  by  the  ectoderm  cells, 
and  the  primordial  germ  cells  may  then  be  recognized 
as  the  anlage  of  the  reproductive  organs. 

Kuhn  discusses  the  origin  and  significance  of  the 
'*Nahrzellenkern "  and  comj)ares  this  body  with 
similar  bodies  which  have  been  found  in  the  jjriinor- 


172        GERM-CELL  CYCLE   IN  ANIMALS 

dial  germ  cells  of  other  animals,  but  is  unable  to  ar- 
rive at  any  final  conclusion. 

In  certain  Cladocera  and  Copepoda,  as  we  have 
seen,  there  are  visible  substances  within  the  cyto- 
plasm of  the  egg  which  become  segregated  in,  and 
render  distinguishable,  the  primordial  germ  cell .  Some 
species  belonging  to  these  and  other  groups  of  Crus- 
tacea have  been  studied  in  which  such  a  visible  sub- 
stance pecuKar  to  the  primordial  germ  cell  is  absent. 

Samassa  (1893)  not  only  failed  to  find  the  pri- 
mordial germ  cell  during  the  cleavage  stages  of 
Moina  rectirostris,  but  claims  that  the  germ  cells 
arise  from  four  mesoderm  cells.  Kuhn  (1908),  from 
a  study  of  the  parthenogenetic  generation  of  Daphnia 
j)ulex  and  Polyphemus  pediculus,  also  derives  the  germ 
cells  from  the  mesoderm.  Vollmer  (1912)  could  not 
distinguish  the  germ  cells  of  Daphnia  magna  and  D. 
pulex  in  the  developing  winter  eggs  until  the  blasto- 
derm was  almost  completed  and  Muller-Cale  (1913) 
could  not  find  these  cells  in  Cypris  incongruens  until 
the  germ  layers  were  fully  formed.  McClendon 
(1906a)  has  shown  that  in  two  parasitic  copepods, 
Pandarus  sinuatus  and  an  unnamed  species,  the  pri- 
mordial germ  cell  is  established  at  the  end  of  the 
fifth  cleavage  (32-cell  stage)  instead  of  at  the  end  of 
the  fourth  as  Haecker  (1897)  found  in  Cyclops.  It 
is  suggested  that  this  delay  may  be  due  to  the  large 
amount  of  yolk  present.  The  stem-cell  from  which 
it  arises  is,  however,  not  made  visibly  different  from 
the  rest  of  the  blastoderm  by  peculiar  granules  as  is 
the  case  in  Cyclops. 


GERM   CELLS   IN   THE   ARTHROPODA     173 

Bigelow  (1902)  has  described  in  Lepa.s  cuiafifera 
and  L.  fascicularis  certain  stages  which  may  Ijring 
the  forms  in  which  no  early  segregation  of  the  germ 
cells  has  been  discovered  into  line  with  the  apparently 
more  determinate  species.  In  Lepas  the  yolk, 
which  at  first  is  evenly  distributed  within  the  egg, 
passes  to  the  vegetative  pole  and  becomes  segregated 
in  one  of  the  first  two  cleavage  cells  {cd~).  At  the 
16-cell  stage  the  yolk  lies  within  the  single  entoblast 
cell  (d^-^),  which  occupies  a  position  corresponding  to 
that  of  the  primordial  germ  cell  in  Moina.  In  this 
connection  may  be  mentioned  the  fact  that  in  many 
animals  the  germ  cells  are  supposed  to  come  from  the 
entoderm  and  are  characterized  by  the  possession 
of  much  yolk. 


CHAPTER  VI 

THE  SEGREGATION  OF  THE  GERM  CELLS  IN  NEM- 
ATODES, SAGITTA,  AND  CERTAIN  OTHER  MET- 
AZOA' 

1.  The  Keimbahn  in  the  Nematoda 

The  classical  example  of  the  keimbahn  in  animals 
is   that   of   Ascaris  megalocephala   as   described   by 
Boveri  (1887,  1892).     The  first  cleavage  division  of 
the  egg  of  Ascaris  results  in  two  daughter  cells,  each 
containing  two  long  chromosomes  (Fig.  51,  A).     In 
the  second   division  the   chromosomes   of   one  cell 
divide  normally  and  each  daughter  cell  receives  one 
half  of  each  (Fig.  51,  B,  S).     The  chromosomes  of 
the  other  cell  behave  differently;    the  thin  middle 
portion  of  each  breaks  up  into  granules  (Fig.  51,  A) 
which  split,  half  going  to  each  daughter  cell,  but  the 
swollen  ends  (Fig.  51,  B,  C)  are  cast  off  into  the  cyto- 
plasm.    In  the  four-cell  stage  there  are  consequently 
two  cells  with  the  full  amount  of  chromatin  and  two 
with   a   reduced    amount.     This   inequality   in   the 
amount  of  chromatin  results  in  different-sized  nuclei 
(Fig.   51,   C) ;    those  with  entire  chromosomes   (S) 
are  larger  than  those  that  have  lost  the  swollen  ends 
(C).     In  the  third  division  one  of  the  two  cells  w^ith 
the  two  entire  chromosomes  loses  the  swollen  ends 
of  each ;  the  other  (Fig.  51,  Z),  8)  retains  its  chromo- 

174 


GERM   CELLS  IN  NEMATODES,  SAGITTA     175 

somes  intact.  A  similar  reduction  in  the  amount  of 
chromatin  takes  place  in  the  fourth  and  fifth  divi- 
sions and  then  ceases.  The  single  cell  in  the  32-cell 
stage  which  contains  the  full  amount  of  chromatin 


Fig.  51.  —  Ascaris.  Stages  in  early  cleavage  showing  the  chromatin- 
diminution  process  in  all  cells  except  the  stem  cell  (S).  {From 
Boveri,  1892.) 

has  a  larger  nucleus  than  the  other  thirty-one  cells 
and  gives  rise  to  all  of  the  germ  cells,  whereas  the 
other  cells  are  for  the  production  of  somatic  cells 
only.  The  cell  lineage  of  Ascaris  is  shown  in  the 
accompanying  diagram  (Fig.  5'i), 


176        GERM-CELL   CYCLE   IN  ANIMALS 

Meyer  (1895)  extended  the  study  of  chromatin- 
diminution  to  other  species  of  Ascaris.  In  A.  lum- 
bricoides  no  diminution  takes  place  until  the  four- 
cell  stage ;  then  three  of  the  nuclei  become  deprived 
of  part  of  their  chromatin.     A  diminution  of  this 


O     O 


Q  O  O  O  O 


o  o    o  o      o 


Fig.  52.  —  Ascaris.  Diagram  showing  segregation  of  primordial  germ 
cell.  E  =  egg  ;  Pi,  P2,  P3  =  stem  cells ;  P4  =  primordial  germ  cell. 
Circles  represent  somatic  cells.     {From  Boveri,  1910.) 

sort  had  been  described  by  Boveri  as  a  variation  in 
the  process  observed  in  A.  megalocephala.  In  A. 
rubicunda  the  differentiation  of  the  cleavage  cells 
seems  to  resemble  A.  megalocephala  more  than  it 
does  A.  lumbricoides.  Only  late  cleavage  stages 
of  A.  labiata  were  obtained  by  Meyer,  but  there  is 


GERM   CELLS  IN   NEMATODES,  SAGITTA     177 

no  doubt  that  a  similar  process  occurs  here.  The 
general  conclusion  is  reached  that  the  cleavage  cells 
of  all  AscARiD^  undergo  a  chromatin  diminution. 

Bonne  vie  (1901),  however,  while  able  to  confirm 
Meyer's  results  so  far  as  A.  lumbricoides  is  concerned, 
could  discover  no  process  of  diminution  in  Strongylus 
paradoxus  and  Rhabdonema  nigrovenosa. 

The  elimination  of  chromatin  from  all  of  the 
somatic  cells  of  Ascaris  and  not  from  the  germ  cells 
led  to  the  conclusion  that  the  germ-plasm  must  re- 
side in  the  chromatin  of  the  nucleus.  The  more 
recent  experimental  investigations  of  Boveri  (1910a, 
19106),  however,  indicate  that  it  is  not  the  chromatin 
alone  that  determines  the  initiation  of  the  diminu- 
tion process,  but  that  the  cytoplasm  plays  a  very  im- 
portant role.  Dispermic  eggs  were  found  to  segment 
so  as  to  produce  three  types  as  follows : 

Type  I,  with  one  stem  cell  (P)  and  three  primordial 
somatic  cells  (AB) ; 

Type  II,  with  two  stem  cells  and  two  primordial 
somatic  cells ;   and 

Type  III  with  three  stem  cells  and  one  primordial 

somatic  cell. 

Fig.  53,  B  shows  a  cleavage  stage  of  Type  II. 
Here  are  represented  two  stem  cells  (P)  with  the  com- 
plete amount  of  chromatin,  both  of  which  are  pro- 
paring  to  divide  to  form  the  stem  cells  (P-z)  of  the  next 
generation.  From  the  study  of  these  dispermic 
eggs  Boveri  (1910)  concludes  ^  that  it  is  ''die  unrich- 

i"Durch    die    simultane    Vicrteilung   eines    dispermen    Ascaris-Eies 
entstehen  (vielleicht  mit  ganz  seltenen  Ansnahmen)  Zellen.  welche  die 
N 


178        GERM-CELL   CYCLE   IN  ANIMALS 

tigen  plasmatischen  Qualitdten  des  sich  entwickeln- 
den  Zellenkomplexes "  that  cause  the  injurious  re- 
sults of  dispermy,  and  that  if,  of  the  three  types 
of  dispermic  eggs  described,  the  cells  could  be  iso- 
lated in  pairs,  one  AB-ceW  paired  with  one  Pi-cell, 


Fig.  53.  —  Ascaris.     A.    Chromatin-diminution   in    a   centrifuged    egg. 
B.  In  a  dispermic  egg.     (From  Boveri,  1910.) 

an  embryo,  normal  except  in  size,  would  result 
from  each  pair. 

Eggs  that  were  strongly  centrifuged  cut  off  at  the 
beginning  of  the  first  cleavage  at  the  heavy  pole  a 

gleiche  Wertigkeit  besitzen,  wie  diejenigen,  die  durch  Zweiteilung  eines 
normal-befruchteten  Eies  gebildet  werden,  namlich  die  Wertigkeit  AB 
oder  Pi.  Es  konnen  drei  Zellen  die  Qualitat  AB  besitzen  oder  zwei  oder 
eine;  dem  jeweiligen  Rest  kommt  die  Qualitat  Pi  zu.  Schon  beim 
Uebergang  vom  vierzelligen  zum  achtzelligen  Stadium  lasst  sich  aus  der 
Teilungsrichtung  mit  sicherheit  diagnostizieren,  welche  der  vier  primaren 
Blastomeren  als  AB,  welche  als  Pi  aufzufassen  sind;  und  diese  Wert- 
bestimmung  wird  durch  die  weiteren  Schicksale  der  vier  Zellfamilien  in 
jeder  Hinsicht  bestatigt  "  (p.  157). 


GERM   CELLS  IN  NEMATODES,  SAGITTA    179 

granular  ball  (Fig.  53,  A,  B).  This  phenomenon  was 
reported  by  Hogue  (1910)  and  such  eggs  were  termed 
"Balleier."  In  these  eggs  the  two  cells  of  the  four- 
cell  stage  which  are  adjacent  to  the  "Ball"  undergo 
the  diminution  process  (Fig.  53,  A,  AB) ;  the  re- 
maining two  are  stem  cells  which  give  rise  to  the 
germ  cells  (Fig.  53,  A,  P).  Thus  there  are  two 
"Keimbahnen"  proceeding  side  by  side  in  a  single 
egg  and  four  primordial  germ  cells  are  produced  in- 
stead of  two  as  in  normal  eggs  (see  Fig.  51).  Miss 
Hogue's  experiments  w^ith  centrifugal  force  led  her 
to  conclude  that  these  must  be  an  "unsichtbare 
Polaritat"  or  "  Protoplasmaachse "  in  the  egg  of  the 
Ascaris.  Boveri  agrees  w^ith  this  and  considers 
further  that  the  initiation  of  the  diminution  process 
is  not  determined  by  the  chromatin  but  by  the 
cytoplasm  of  the  egg.^ 

2.  The  Keimbahn  in  Sagitta 

Sagitta  has  proved  to  be  of  considerable  impor- 
tance to  those  interested  in  the  keimbahn  of  animals. 
Hertwig  (1880)  figures  the  four  primitive  germ  cells 
in  the  gastrula  and  later  stages,  proving  that  these 
cells  are  early  set  aside  in  embryonic  development. 
Recently  the  work  of  Elpatiewsky  (1909,  1910)  has 

1  He  states  that,  "Was  aber  aiich  hier  durch  weitere  Untersuchungen 
noch  erreicht  werden  mag,  Eines  halto  ich  fur  sicher,  dass  sich  alios,  was 
uber  die  Wertigkcit  der  priiniiron  Blastoiiicron  bei  abnonner  Fiirchung 
ermittelt  worden  ist,  durch  die  Annahrae  sehr  einfacher  Plasmadifferenzen 
erklaren  lasst,  wogegen  die  HyixUhese  cinor  differenzierenden  Wirkung 
des  Kerns  in  jeder  Form  auf  imUberwindliche  Schwierigkeiten  stosst " 
(p.  206). 


180        GERM-CELL  CYCLE  IN  ANIMALS 


given  Sagitta  a  new  importance,  since  this  writer  has 
found  within  the  fertiHzed  egg  a  cytoplasmic  inclu- 
sion which-  is  intimately  associated  with  the  segre- 
gation of  the  germ  cells.  The  presence  of  this  inclu- 
sion has  been  confirmed  by  Buchner  (1910a,  19106) 


'^^^^m^M:^^::^. 


C  D  ^  fr 

Fig.  54.  —  Sagitta.  A.  First  appearance  of  the  "  besondere  Korper" 
(bK)  in  the  egg.  B.  Egg  with  germ  nuclei  fusing.  X  =  "  besondere 
Korper."  C.  Thirty-two-cell  stage;  the  primordial  germ  cell  (G) 
contains  the  "besondere  Korper"  (X).  D.  Two  entoderm  cells  (E) 
and  dividing  primordial  germ  cell.  E.  Two  primordial  germ  cells 
showing  unequal  distribution  of  "besondere  Korper"  (X).  F.  Di- 
vision of  first  two  primordial  germ  cells ;  one  dividing  more  rapidly 
than  the  other.     (Frorti  Elpatiewsky,  1909,  1910.) 

and  Stevens  (19106),  and  several  ideas  have  been 
expressed  regarding  its  origin,  fate,  and  significance. 
Elpatiewsky  (1909)  found  in  Sagitta,  at  the  time 
when  the  male  and  female  nuclei  were  lying  side  by 
side  in  the  middle  of  the  egg,  a  body  situated  near 


GERM   CELLS   IN   NEMATODES,   SAGITTA     181 

the  periphery  at  the  vegetative  pole  (Fig.  54,  B,  x). 
This  body,  which  he  called  the  *'besondere  Korper," 
consists  at  first  of  "  grobkornigeri "  plasma  which 
stains  like  chromatin  but  not  so  intensely ;  later  it 
condenses  into  a  round  homogeneous  body  with  a 
sharp  contour.  During  the  first  five  cleavage 
divisions  the  "besondere  Korper"  is  always  confined 
to  a  single  cell.  At  the  completion  of  this  fifth 
cleavage  (32-cell  stage),  the  blastomere  containing 
this  cytoplasmic  inclusion  is  recognizable  as  the  first 
*'Urgeschlechtszelle"  (Fig.  54,  C,  G)  and  its  larger 
sister  cell  as  the  first  **Urentodermzelle"  (Fig.  54, 
Cy  E).  The  primordial  germ  cell  is  the  last  to  divide 
during  the  sixth  cleavage  and  the  "besondere  Kor- 
per" does  not,  as  before,  pass  entire  into  one  of  the 
daughter  cells,  but  breaks  up  into  a  number  of  pieces, 
part  of  which  are  included  in  each  of  the  two  daughter 
cells  (Fig.  54,  D,  X).  One  of  these  daughter  cells 
apparently  acquires  more  of  the  *'besondere  Korper" 
than  the  other.  This  division  appears  to  Elpatiew- 
sky  to  be  differential,  separating  the  primordial 
oogonium  from  the  primordial  spermatogonium,  the 
latter  being  the  cell  which  receives  the  larger  portion 
of  the  "besondere  Korper"  and  which  during  the 
next  (seventh)  division  is  sHghtly  delayed  (Fig.  54, 
F).  Subsequent  to  the  seventh  cleavage  the  remains 
of  the  "besondere  Korper"  become  pale  and  grad- 
ually disappear,  apparently  dissolving,  and  in  the 
four  germ  cells  resulting  from  the  next  division  only 
occasionally  can  stained  granules  from  this  body  be 
distinguished. 


182         GERM-CELL   CYCLE   IN  ANIMALS 

Buchner  (1910a,  19106)  had  no  difficulty  in  find- 
ing the  "besondere  Korper"  of  Elpatiewsky  and  in 
tracing  it  during  the  cleavage  stages.  He  claims  that 
it  originates  from  the  "accessory  fertilization  cell" 
described  by  Stevens  (1904)  as  degenerating  after 
the  egg  breaks  away  from  the  oviduct  wall,  and  that 
it  is  chromidial  in  nature  and  should  therefore  be 
called  *'Keimbahnchromidien."  Stevens  (1910), 
however,  has  carefully  examined  abundant  material 
from  Sagitta  elegans  and  S.  bipunctata,  and  no  connec- 
tion between  the  "accessory  fertilization  cell"  and 
the  "besondere  Korper"  could  be  traced,  the  latter 
appearing  for  the  first  time  at  the  stage  when  the 
egg  and  sperm  nuclei  lie  side  by  side  in  the  middle 
of  the  egg,  thus  confirming  Elpatiewsky's  conclusions. 
She  admits  the  possibiHty  of  the  origin  of  the  "be- 
sondere  Korper"  from  granules  of  the  accessory 
fertiHzation  cell,  provided  this  material  loses  its  stain- 
ing capacity  for  a  period,  and  suggests  also  that  the 
granules  of  chromatin-like  material  extruded  from 
the  nucleus  of  the  egg  during  maturation  may  take 
part  in  its  formation.  Miss  Stevens  also  beheves 
with  Elpatiewsky  that  the  "besondere  Korper" 
divides  unequally  between  the  two  daughter  cells  of 
the  primordial  germ  cell  and  that  this  is  a  differential 
division.  She  was  unable,  however,  to  detect  any 
constant  difference  between  either  the  cytoplasm  or 
the  nuclei  of  oogonia  and  spermatogonia.  It  is 
worthy  of  mention  that  Elpatiewsky  (1910)  believes 
that  the  "besondere  Korper"  may  originate  "aus 
dem  achromatischen  Kernkorper." 


GERM   CELLS  IN  NEMATODES,  SAGITTA     18S 

3.  The  Keimbahn  in  Other  Metazoa 

Certain  phenomena  have  been  reported  in  the 
early  development  of  the  eggs  of  many  other  animals 
which  have  either  been  compared  or  can  be  compared 
with  conditions  such  as  we  have  described  in  the 
preceding  portions  of  this  book. 

The  large  nucleolus  in  the  germinal  vesicle  of  the 
medusa,  Mquorea  forskalea  (Fig.  55,  A),  according 
to  Haecker  (1892),  disappears  from  the  germinal 
vesicle  about  half  an  hour  after  the  egg  is  laid,  and 
a  similar  body  becomes  evident  near  the  egg  nucleus 
which  has  in  the  meantime  become  smaller  (Fig.  55, 
B).  These  two  bodies  are  considered  by  Haecker 
to  be  identical,  and  the  term  ''Metanucleolus"  has 
been  applied  to  them.  The  metanucleolus  is,  in  each 
division  up  to  the  sixty-four  cell  stage,  segregated 
intact  in  one  cell.  Its  further  history  was  not 
traced,  but  in  the  blastula  (Fig.  55,  D)  when  the  colls 
at  the  posterior  pole  begin  to  differentiate,  nucleolar- 
like  bodies  appear  in  some  of  them  which  are  absent 
from  the  undifferentiated  blastula  elements.  These 
may  be  the  descendants  of  the  metanucleolus. 

A  body  similar  to  the  metanucleolus  was  also  dis- 
covered by  Haecker  near  the  copulating  germ  nuclei 
in  the  egg  of  Aurelia  aurita,  but  its  history  could  not 
be  determined  because  of  the  large  amount  of  yolk 
present.  Haecker  identifies  the  metanucleolus  of 
Mquorea  with  the  spherical  body  described  hy  Metcli- 
nikoff  (1886)  near  the  egg  nucleus  of  M ifroconia 
annoe,  and   considered  by  him  as  a  sperm   nucleus. 


184        GERM-CETJ.   CYCLE   IN   ANIMALS 

A  similar  interpretation  is  given  by  Haecker  for  the 
cytoplasmic    inclusion    ("Spermakern")    found    by 

Boveri  (1890)  in 
Tiara.  Similarly 
the  ''Kleinkern" 
which  Chun  (1891) 
discovered  in  the 
egg  cells  of  Ste- 
phanophyes  su- 
perba,  and  the 
bodies  described 
by  Hertwig  (1878) 
near  the  matura- 
tion spindles  of 
Mytilus  and  Sa- 
gitta,  resemble 
very  closely  the 
metanucleolus  of 
^quorea. 

Furthermore, 
the  metanucleolus 
is   considered   by 
Haecker  homolo- 
gous to  the  "Par- 
acopulationszelle 
described  by  Weis- 
mann  and  Ischi- 
kawa  in  the  winter 
eggs  of  certain 
Daphnid^,  and  in  both  cases  it  is  considered  prob- 
able that  these  peculiar  bodies  are  restricted  to  the 
*'Keimbahnzellen"  of  the  embryo. 


Fig.  55.  —  A-D.  Stages  in  formation  of 
blastula  of  jEquorea  forskalea  showing  seg- 
regation of  metanucleolus.  {From  Haecker, 
1892.)  E.  Oocyte  of  the  cat  containing 
the  "corps  enigmatique"  (c.e).  {From 
Vander  Stricht,  1911.) 


GERM   CELLS  IN  NEMATODES,  SAGITTA     185 

In  the  eggs  of  Myzostoma,  Wheeler  (1897)  found 
that  the  nucleolus  of  the  germinal  vesicle  does  not 
dissolve  soon  after  it  is  cast  out  into  the  cytoplasm 
during  the  formation  of  the  first  maturation  spindle, 
but  remains  visible  at  least  until  the  eight-cell  stage, 
at  which  time  it  lies  in  the  large  posterior  macromere, 
a  cell  which  "very  probably  gives  rise  to  the  entoderm 
of  the  embryo."  Later  embryonic  stages  were  not 
studied.  According  to  Wheeler  "the  nucleoli  are 
relegated  to  the  entoderm  cells  as  the  place  where 
they  would  be  least  liable  to  interfere  in  the  further 
course  of  development  and  where  they  may  perhaps 
be  utilized  as  food  material  after  their  disintegra- 
tion "  (p.  49). 

McClendon  (19066)  has  likewise  described  a  body 
embedded  in  the  cytoplasm  of  the  egg  of  Myzostoma 
clarki  which  he  derives  from  the  "accessory  cells" 
which,  as  Wheeler  (1896)  has  shown,  attach  them- 
selves to  either  pole  of  the  oocytes.  These  "acces- 
sory" cells  are  really  the  "Nahrzellen"  of  other 
authors.  The  cleavage  of  the  egg  was  not  studied. 
Buchner  (19106)  suggests  that  this  body  described 
by  McClendon  and  the  "nucleolus"  of  Wheeler  are 
identical  and  that  through  them  the  keimbahn  may 
be  determined. 

Granules  of  various  sorts  have  been  noted  in  the 
eggs  of  various  animals  which  are  segregated  in  par- 
ticular blastomeres  and  may  have  some  relation  to 
the  keimbahn.  For  example,  among  the  mollusks, 
Blockmann  (1881)  has  described  the  appearance  of 
a  group  of  granules  in  the  early  cleavage   cells   of 


186        GERM-CELL  CYCLE   IN  ANIMALS 

Neritina  which  finally  reach  the  velar  cells.  It  is 
also  probable  that  Fol  (1880)  observed  similar  gran- 
ules in  the  16-cell  stage  of  Planorhis.  In  the  same 
category,  no  doubt,  belong  the  bodies  figured  by 
Fujita  (1904)  in  the  4-cell  to  the  16-cell  stages  of 
Siphonaria  lying  at  the  vegetative  pole,  and  the 
"Ectosomen"  described  and  figured  by  Wierzejski 
(1906)  in  Physa.  These  granules  appear  at  the  vege- 
tal pole  in  the  blastomeres  of  Physa  during  the 
second  cleavage ;  are  at  first  embedded  in  the  ento- 
derm mother  cells,  but  finally  become  localized  in 
the  ectoderm  cells.  They  periodically  appear  and 
disappear,  and  may,  as  suggested  by  Wierzejski, 
represent  only  "eine  besondere  Erscheinung  des 
Stoffwechsels"  (p.  536). 

Similarly  in  the  rotifer,  ^52?Zanc/ma,  Jennings  (1896) 
has  traced  a  "cloud  of  granules"  from  the  eight-cell 
stage  until  the  seventh  cleavage,  when  this  mass 
forms  part  of  the  smaller  entodermal  cell.  In  Lejpas 
there  has  also  been  recorded  (Bigelow,  1902)  a  segre- 
gation of  granules  in  one  blast omere.  Many  other 
substances  granular  in  form  have  been  described  in 
the  eggs  of  animals,  some  of  them  at  least  having 
migrated  there  from  the  somatic  tissue.  Blockmann 
(1887)  discovered  a  number  of  bacteria-like  rods 
in  the  undeveloped  eggs  of  Blatta  germanica;  these 
rods  multiplied  by  division  and  were  considered  sym- 
biotic bacteria.  "  Bacterienartige  Stabchen"  were 
also  noted  by  Heymons  (1895)  in  the  eggs  of  Peri- 
planata  orientalis  and  Ectobia  livida;  these  sink  into 
the  yolk  and  disappear.     More  recently  a  report  of 


GERM   CELLS  IN  NEMATODES,   SAGITTA     187 

Buchner  (1912)  indicates  that  these  bodies  are  really 
organisms  which  seem  to  be  symbiotic  and  not  para- 
sitic, although  it  remains  to  be  proved  what  advan- 
tage the  host  receives  from  their  presence.  Of  a 
similar  sort  are  the  Zooxanthellse  which  Mangan 
(1909)  has  shown  enter  the  developing  ovum  from 
the  parental  tissues.  All  of  these  organisms  become 
in  some  way  embedded  in  the  germ  cells,  but  so  far 
as  we  know  never  serve  to  distinguish  the  keimbahn, 
although  a  more  selective  distribution  within  the 
developing  animal  would  obviously  be  greatly  to 
their  advantage. 

Vander  Stricht  (1911)  has  compared  the  *'beson- 
dere  Korper"  found  by  Elpatiewsky  (1909,  1910) 
in  the  egg  of  Sagitta  with  several  bodies,  the  *' corps 
enigmatique,"  which  he  discovered  in  the  oocyte  of 
the  cat  (Fig.  55 y  E).  One  or  two  of  these  *' corps 
enigmatique"  are  present  in  the  young  oocyte 
originating  from  a  few  (one  to  five)  cytoplasmic 
safranophile  granules  which  are  visible  at  the  begin- 
ning of  the  growth  period.  They  at  first  lie  near  the 
nucleus,  but  as  the  size  of  the  oocyte  increases  they 
become  situated  near  the  periphery.  Usually  three 
parts  can  be  recognized  in  the  *' corps  enigmatique"  : 
*' granulation  centrole,  couche  intermediaire  et  couche 
corticale  foncee."  As  the  term  applied  to  them  indi- 
cates, the  functions  of  these  bodies  were  not  deter- 
mined. The  following  suggestion  is,  however,  made  : 
**il  est  possible  que  cet  element  nous  montre,  des 
Forigine,  la  'Keimbahn'  ainsi  que  les  premieres 
cellules    genitales    constituees."      A    body    stained 


188         GERM-CELL   CYCLE   IN  ANIMALS 

deeply  by  nuclear  dyes  which  was  found  by  O.  Van 
der  Stricht  (1909)  in  the  bat  at  the  time  of  the  first 
cleavage  mitosis  may  be  similar  to  the  "corps  enig- 
matique"  of  the  cat. 

In  many  animals  no  keimbahn-determinants  nor 
similar  bodies  have  as  yet  been  discovered.  The  best 
we  can  do  in  cases  of  this  sort  is  to  determine  from 
what  cleavage  cell  or  cells  the  germinal  epithelium 
probably  originates.  For  example,  in  Arenicola, 
Lillie  (1905)  has  shown  that  the  part  of  the  perito- 
neum from  which  the  germ  cells  arise  develops  from 
teloblast  cells  which  are  probably  derived  (Child, 
1900)  from  cell  4;d,  At  present,  however,  no  charac- 
teristics have  been  discovered  which  enable  us  to 
distinguish  between  the  germ  cells  and  the  somatic 
cells  in  the  early  embryonic  stages  of  such  animals 
(Downing,  1911). 


CHAPTER  VII 

THE  GERM  CELLS  OF  HERMAPHRODITIC  ANIMALS 

Many  of  the  most  interesting  biological  problems 
are  those  connected  with  the  phenomenon  of  sex. 
The  term  **sex"  is  applied  to  the  soma  or  body  of  an 
organism;  it  indicates  the  presence  of  certain  mor- 
phological and  physiological  characteristics,  which 
may  be  separated  into  primary  and  secondary  sexual 
characters.  The  primary  sexual  characters  are  those 
immediately  connected  with  the  reproductive  organs ; 
the  secondary  sexual  characters,  such  as  the  beard  of 
man,  the  brilliant  feathers  and  beautiful  songs  of 
many  male  birds,  and  the  antlers  of  the  moose,  repre- 
sent differences  between  male  and  female  individuals 
not  directly  concerned  with  the  production  of  germ 
cells.  It  is  customary  to  speak  of  male  germ  cells 
and  female  germ  cells;  this  is  not  strictly  proper, 
since  in  only  a  few  special  cases  can  we  predict  the 
sex  of  the  individual  which  will  develop  from  an  egg. 
Moreover,  every  germ  cell  must  contain  the  poten- 
tiality of  both  sexes  since  sooner  or  later  its  descend- 
ants will  give  rise,  some  to  male,  some  to  female  or 
perhaps  to  hermaphroditic  offspring.  Thus  the  egg 
is  an  initial  hermaphrodite ;  it  may  or  may  not  be- 
come an  eventual  hermaphrodite  according  to  the  sex- 
ual condition  of  the  individual  to  which  it  gives  rise. 

189 


190        GERM-CELL   CYCLE   IN  ANIMALS 

All  the  species  of  Metazoa  may  be  separated  into 
two  groups.     The  individuals  in  one  group  of  species 


Fig.  56.  —  Diagram  of  the  reproductive  organs  of  the  earthworm,  dorsal 
view.  A,  B,  C,  seminal  vesicles  ;  N,  nerve-cord  ;  O,  ovary ;  OD,  ovi- 
duct ;  R,  egg  sac ;  S,  spermatheca ;  SF,  seminal  funnel ;  T,  testes ; 
VD,  vas  deferens.     {From  Marshall  and  Hurst.) 

possess  only  one  sort  of  reproductive  organs  (male  or 
female)  and  produce  only  one  sort  of  germ  cells  (eggs 
or  spermatozoa) ;    these  species  are  said  to  be  dice- 


GERM   CELLS   OF  HERMAPHRODITES     191 

cious  or  gonochoristic.  In  the  other  group  both 
male  and  female  reproductive  organs  occur  in  each 
individual ;  and  such  species  are  called  monoecious 
or  hermaphroditic.  The  majority  of  animals  are 
gonochoristic,  but  a  number  of  classes  and  orders 
consist  almost  entirely  of  hermaphroditic  species, 
and  probably  no  large  group  of  animals  is  free  from 
species  which  are  monoecious.  A  study  of  hermaph- 
roditism is  necessary  for  the  elucidation  of  many 
biological  problems ;  and  some  of  those  dealing 
more  directly  with  the  germ-cell  cycle  will  be  con- 
sidered in  this  chapter. 

There  are  many  variations  in  the  morphology  of 
the  reproductive  organs  in  hermaphrodites.  In 
some,  such  as  the  earthworm  (Fig.  56),  the  male 
and  female  organs,  consisting  of  all  the  parts  typically 
present  in  gonochoristic  animals,  are  present  and 
entirely  separate  from  each  other.  All  gradations 
between  such  a  state  and  an  intimate  association  of 
male  and  female  germ  cells  are  known.  Perhaps  the 
most  interesting  series  occurs  among  the  mollusks. 
Here  the  germ  gland  may  consist  of  two  regions,  as 
in  Pecten  maximus,  one  of  which  gives  rise  to  ova,  the 
other  to  spermatozoa ;  or  certain  cysts  may  contain 
only  female  germ  cells  and  other  cysts  only  male 
germ  cells,  or  both  sorts  of  germ  cells  may  occur  in  a 
single  cyst. 

Hermaphroditism  has  been  shown  to  be  prevalent 
among  animals  that  are  parasitic  or  sedentary,  or  for 
some  other  reason  may  become  isolated  from  their 
fellows.     Thus,  it  is  of  advantage  for  a  parasite,  such 


192        GERM-CELL   CYCLE   IN  ANIMALS 

as  the  tapeworm,  to  be  able  to  form  both  male  and 
female  germ  cells,  since  it  may  at  any  time  become  the 
only  one  of  its  species  to  occupy  the  alimentary  canal 
of  a  host.  Hermaphroditism  in  such  a  case,  however, 
is  of  no  benefit  if  self-fertilization  is  not  possible. 
Although  there  are  thousands  of  hermaphroditic 
species  of  animals  there  are  comparatively  few  whose 
eggs  are  known  to  be  fertilized  by  spermatozoa  from 
the  same  individual.  We  must  therefore  distinguish 
between  morphological  and  physiological  hermaphro- 
ditism and  recognize  the  fact  that  the  former  condi- 
tion is  much  more  prevalent  than  the  latter.  Among 
the  species  in  which  self-fertilization  normally  occurs 
are  certain  rhabdocoels,  digenetic  trematodes,  ces- 
todes,  ascidians,  and  mollusks.  Van  Baer,  in  1835, 
claims  to  have  observed  self-copulation  in  the  snail, 
Lymncea  auricularia;  that  is,  an  individual  with  its 
penis  inserted  in  its  own  female  opening.  That 
species  of  this  genus  fertilize  their  own  eggs  has 
frequently  been  stated  by  investigators.  Frequently 
the  spermatozoa  of  an  hermaphrodite  are  capable 
of  fertilizing  the  eggs  of  the  same  individual, 
but  penetrate  more  readily  the  eggs  of  other  individ- 
uals. Such  is  the  case  in  the  ascidian,  Ciona  in- 
testinalis  (Castle,  1896;   Morgan,  1905). 

Both  sorts  of  germ  cells  are  seldom  produced  at 
the  same  time  by  hermaphrodites.  Those  species 
in  which  spermatozoa  mature  first  are  called  protan- 
dric;  this  is  the  usual  condition.  In  a  few  cases 
eggs  are  formed  first  and  later  spermatozoa;  in- 
dividuals in  which  this  occurs  are  called  protogynic. 


GERM   CELLS   OF  HERMAPHRODITES     193 

Proterogyny  has  been  described  in  certain  ascidians 
(Salpa),  pulmonale  gasteropods,  and  corals.  That 
hermaphrodites  are  not  sexless  but  really  animals 
with  double  sex  is  well  shown  by  the  life  history  of 
a  worm,  Myzostoma  pulminar,  which  passes  through 
a  short  male  stage  during  which  spermatozoa  are 
produced,  then  a  stage  when  no  functional  germ  cells 
are  formed,  and  finally  a  female  stage,  characterized 
by  the  development  of  eggs  (Wheeler,  1896).  Thus, 
in  this  species,  although  hermaphroditic,  there  is  no 
functional  hermaphroditic  stage.  All  variations  be- 
tween this  entire  separation  of  the  periods  of  germ- 
cell  development  and  the  simultaneous  production 
of  male  and  female  germ  cells  have  been  recorded. 
Some  degree  of  protandry  has  been  observed  among 
the  sponges,  coelenterates,  flat  worms,  segmented 
round-worms,  mollusks,  echinoderms,  Crustacea,  and 
chordates. 

Hermaphroditism  may  occur  in  only  a  few  families, 
genera,  or  species  in  a  class.  This  is  true,  for  example, 
among  the  anthropods  and  vertebrates.  Normally 
the  insects  are  called  dioecious,  but  among  bees,  ants, 
and  butterflies,  and  more  rarely  other  groups,  individ- 
uals appear  which  exhibit  male  characters  on  one  side 
of  the  body  and  female  characters  on  the  other,  or  the 
anterior  part  may  be  male,  the  posterior  female,  etc. 
(von  Siebolt,  1864  ;  Schultze,  1903 ;  Morgan,  1907, 
1913).  Such  a  phenomenon  is  known  as  gynan- 
dromorphism.  Several  hypotheses  have  been  pro- 
posed to  account  for  this  condition.  Boveri  has 
suggested  that  if  the  egg  nucleus  should  chance  to 


194        GERM-CELL  CYCLE  IN  ANIMALS 

divide  before  the  sperm  nucleus  fuses  with  it,  the 
latter  may  unite  with  one  of  the  daughter  nuclei 
of  the  egg  nucleus ;  this  cell  with  this  double  nucleus 
might  then  produce  female  structures,  whereas  the 
other  cell  with  only  a  single  nucleus  representing  one- 
half  of  the  egg  nucleus  might  give  rise  to  male  char- 
acters. Morgan  has  proposed  another  theory  which 
is  based  on  the  fact  that  more  than  one  spermatozoon 
is  known  to  penetrate  the  eggs  of  insects.  If  one 
of  these  supernumerary  spermatozoa  should  chance 
to  divide,  it  might  result  in  the  formation  of  male 
structures,  whereas  the  cells  containing  descendants 
of  the  egg  nucleus  fused  with  another  sperm  nucleus 
would  exhibit  female  characteristics. 

There  is  some  evidence  that  true  hermaphroditism 
may  exist  among  insects,  at  least  during  their  embry- 
onic and  larval  stages.  Thus  Heymons  (1890)  has 
described  in  a  young  larva  of  the  cockroach,  Phyllo- 
dromia  gervianica,  what  appear  to  be  rudimentary 
egg-tubes,  and  in  another  larva  eggs  were  found  in 
the  testes  which  resembled  those  present  in  the  egg- 
tubes  of  female  larvae  of  the  same  size  (1  mm.  in 
length).  More  recently,  Schonemund  (1912)  has 
reported  the  presence  of  egg-tubes  attached  to  the 
anterior  end  of  the  testes  of  stone-fly  nymphs  {Perla 
marginata) . 

True  hermaphroditism  is  rare  in  man  and  other 
mammals,  but  several  cases  have  been  described  in 
the  pig  by  Sauerbeck  (1909)  and  Pick  (1914),  and  in 
man  by  Simon  (1903),  Uffreduzzi  (1910),  Gudernatsch 
(1911),  and  Pick  (1914). 


GERM   CELLS   OF   HERMAPHRODITES     195 

One  of  the  problems  connected  with  hermaphrodit- 
ism that  has  caused  a  great  amount  of  discussion  is 
whether  the  dioecious  or  the  monoecious  condition 
is  the  more  primitive.  The  majority  of  zoologists 
are  inclined  to  consider  the  hermaphroditic  condition 
more  primitive,  but  a  number  of  careful  investigators 
have  decided  in  favor  of  gonochorism.  Among  these 
are  Delage  (1884),  F.  Muller  (1885),  Pelseener  (1894), 
Montgomery  (1895,  1906),  and  Caullery  (1913). 

Very  little  is  known  regarding  the  segregation  and 
early  history  of  the  germ  cells  of  hermaphrodites. 
The  principal  results  have  been  obtained  from  studies 
on  Sagitta  by  Elpatiewsky  (1909),  Stevens  (19106), 
and  Buchner  (1910a,  19106),  and  on  Helix  by  Ancel 
(1903),  Buresch  (1911),  and  Demoll  (1912).  Boveri 
(1911),  Schleip  (1911),  and  Kruger  (1912)  have  made 
some  interesting  discoveries  on  the  chromosome 
cycle  in  nematodes,  and  likewise  Zarnik  (1911)  on 
pteropod  mollusks.  To  this  list  we  may  add  such 
investigations  as  those  of  King  (1910),  Kuschake- 
witsch  (1910),  and  Champy  (1913),  on  amphibians. 

The  segregation  of  the  germ  cells  in  Sagitta  was 
described  and  figured  in  Chapter  VI  (Fig.  54) .  Here 
the  first  division  of  the  primordial  germ  cell  is  probably 
differential;  one  daughter  cell  becomes  the  ancestor 
of  all  the  ova,  the  other  of  all  the  spermatozoa  in  the 
hermaphroditic  adult.  None  of  the  three  investi- 
gators who  have  studied  this  subject  in  Sagitta  have 
been  able  to  discover  with  certainty  any  visible  differ- 
ences between  the  first  two  germ  cells,  but  Elpatiew- 
sky thinks  the  peculiar  cytoplasmic  inclusion,  called 


196        GERM-CELL   CYCLE   IN  ANIMALS 

by  him  the  "besondere  Korper,"  may  be  unequally 
distributed  between  these  cells,  and  that  the  one 
which  procures  the  larger  portion  is  the  progenitor 
of  the  spermatozoa,  the  other  of  the  ova.  The  evi- 
dence for  this  view  is,  however,  insufficient. 

In  Helix  both  eggs  and  spermatozoa  originate  in 
every  acinus  of  the  ovo-testis ;  it  is  therefore  an  ex- 
cellent species  for  the  study  of  the  differentiation  of 
the  sex  cells.  According  to  Ancel  (1903)  the  anlage 
of  the  hermaphroditic  gland  of  Helix  pomatia  appears 
a  few  hours  before  the  larva  hatches ;  it  consists  of  a 
group  of  cells  situated  in  the  midst  of  the  mesoderm, 
from  which  germ  layer  it  seems  to  originate.  It 
soon  loses  its  rounded  form  and  becomes  elongate; 
then  a  lumen  appears  wdthin  it,  thus  changing  it  into 
a  vesicle  whose  wall  consists  of  a  single  layer  of  cells 
—  a  true  germinal  epithelium.  Secondary,  tertiary, 
etc.,  vesicles  bud  off  from  the  single  original  vesicle, 
forming  the  acini  of  the  fully  developed  gland.  Cel- 
lular differentiation  takes  place  by  the  transformation 
of  the  germinal  epithelial  cells  into  male,  nurse,  and 
female  elements.  An  indifferent  epithelial  cell  is 
shown  in  Fig.  57,  A  ;  the  chromatin  granules  are  con- 
densed to  form  irregular  clumps.  Some  of  these 
indifferent  epithelial  cells  increase  in  size  and  give 
rise  to  indifferent  progerminative  cells ;  the  chroma- 
tin clumps  fuse,  forming  more  or  less  spherical  masses 
(Fig.  57,  E).  From  cells  of  this  sort  originate  both 
the  oogonia  and  spermatogonia.  The  progermina- 
tive male  cell  passes  through  the  stages  shown  in 
Fig.  57,  B-D ;  part  of  the  chromatin  of  the  progermi- 


GERM   CELLS   OF   HERMAPHRODITES     107 

native  cell  loses  its  affinity  for  nuclear  dyes ;  the 
chromatin  masses  become  less  numerous  and  more 
nearly  spherical ;  and  the  entire  cell  increases  in  size, 
the  nucleus  growing  much  more  than  the  cytoplasm. 
These  progerminative  male  cells  divide  mitotically 


Fig.  57.  —  Helix  pomatia.  Stages  in  differentiation  of  male  and  female 
sex  cells  from  indifferent  cells.  A.  Epithelial  indifferent  cell. 
E.  Progerminative  indifferent  cell.  B-D.  Stages  in  transformation 
of  progerminative  cell  into  a  spermatogonium.  F—G.  Stages  in 
transformation  of  progerminative  cell  into  an  oocyte.  (From  Ancel, 
1903.) 


and  then  pass  into  the  lumen  of  the  acinus,  where 
they  may  be  recognized  as  spermatogonia  of  the 
first  order. 

After  the  spermatogonia  have  passed  into  the 
lumen  of  the  acinus  the  wall  is  seen  to  consist  of  two 
groups  of  cells  ;  those  of  one  group  are  central  and  in 
contact  with  the  spermatogonia,  the  others  are  periph- 


198        GERM-CELL   CYCLE   IN   ANIMALS 

eral.  The  centrally  situated  cells  now  increase  in 
size ;  but  their  nuclei  retain  the  original  condition ; 
that  is,  the  chromatin  is  present  in  irregular  clumps. 
These  are  nurse  cells.  After  the  nurse  cells  have 
formed,  certain  of  the  peripheral  cells  increase  in 
volume  and  pass  through  an  indifferent  progermina- 
tive  stage  (Fig.  57,  E).  Then  they  transform  into 
female  progerminative  cells,  as  shown  in  Fig.  57,  F,  G. 
The  chromatin  clumps  break  up  and  become  oriented 
near  the  nuclear  membrane,  where  they  form  a  layer 
of  more  or  less  rounded  bodies  bearing  chromatic 
filaments.  In  the  meantime,  both  nucleus  and  cyto- 
plasm increase  in  amount,  especially  the  cytoplasm. 
This  (Fig.  57,  G)  represents  an  oocyte,  which  does  not 
divide  before  maturation. 

Ancel  concludes  from  these  observations  that  there 
are  three  successive  periods  of  cellular  differentiation 
in  the  hermaphroditic  gland  of  Helix:  (1)  the  ap- 
pearance of  spermatogonia,  (2)  nurse  cells,  and 
(3)  oocytes.  Both  spermatogonia  and  oocytes  pass 
through  the  indifferent  progerminative-cell  stage,  but 
the  nurse  cells  do  not ;  there  are  therefore  two  sorts 
of  differentiation  of  the  indifferent  epithelial  cells. 
Regarding  the  cyto-sexual  determination,  the  follow- 
ing hypothesis  is  advanced  :  A  progerminative  in- 
different cell  becomes  a  male  or  female  element 
according  to  its  environment  at  the  time  of  its  trans- 
formation ;  if  it  appears  before  the  nurse  cells  are 
formed  it  becomes  a  spermatogonium ;  if  nurse  cells 
are  already  present  it  grows  into  an  oocyte.  The 
discovery  of  certain  individuals  containing  only  male 


GERM   CELLS   OF  HERMAPHRODITES       199 

elements  is  explained  by  Ancel   by  supposing  the 
transformation  of  the  cells  into  sex  cells  to  cease 


Fig.  58.  —  Helix  arhustorum.  Stages  in  the  differentiation  of  male  and 
female  sex  cells.  A.  Nucleus  of  germinal  epithelium.  B.  Nucleus 
of  nurse  cell.  C.  Nucleus  of  indifferent  sex  cell.  D.  Spermatogo- 
nium of  first  order.  E.  Spermatogonium  of  second  order.  F.  Grow- 
ing oocyte.     {From  Buresch,  1911.) 

before  nurse  cells  are  formed ;  thus  all  the  sex  cells 
would  become  spermatogonia. 

More  recently  Buresch   (1911)   has  repeated  the 


200        GERM-CELL  CYCLE   IN  ANIMALS 

work  of  Ancel,  using  Helix  arbustorum  for  his  materiaL 
He  confirms  many  of  Ancel's  results,  objects  to  others, 
and  adds  certain  new  observations.  The  germinal 
epithelium  is  considered  by  Buresch  to  be  a  syncy- 
tium containing  both  in  young  and  old  specimens 
three  sorts  of  cells,  indifferent  cells,  egg  cells,  and 
nurse  cells.  Likewise  spermatogonia  are  present 
not  only  in  young  but  also  in  fully  developed  her- 
maphroditic glands.  This  is  contrary  to  Ancel's  idea 
of  successive  transformation.  Buresch' s  view  is 
indicated  in  Fig.  59.  Here  the  vertical  row  of  circles 
represents  the  nuclei  of  the  syncytial  germinal  epithe- 
lium, some  of  which,  as  at  m,  change  to  indifferent 
germ  cells.  These  may  pass  into  the  lumen  of  the 
acinus  as  spermatogonia  of  the  first  order  {Sg.  I) 
and  divide  to  form  spermatogonia  of  the  second  order 
(Sg.  II)  which  grow  into  spermatocytes  (Sc) ;  sper- 
matozoa are  derived  from  these  in  the  usual  manner. 
Other  indifferent  germ  cells  remain  in  the  wall,  as  at 
Wy  and  grow  into  oocytes,  and  a  third  class  of  cells 
become  nurse  cells  {71).  In  Fig.  58,  A  is  shown  a 
nucleus  of  the  germinal  epithelium  about  4  microns 
by  6  microns  in  size.  During  differentiation  into  an 
indifferent  germ  cell  (Fig.  58,  C)  the  chromatin  forms 
a  nucleolus,  and  both  nucleus  and  nucleolus  increase 
in  size  until  the  former  reaches  a  diameter  of  about  7 
microns.  Those  indifferent  germ  cells  that  are  to 
produce  spermatozoa  separate  from  the  epithelium 
with  a  small  amount  of  cytoplasm  and  fall  into  the 
lumen  of  the  acinus  as  spermatogonia  of  the  first 
order  (Fig.  58,  D).     These  divide  to  form  spermato- 


GERM   CELLS   OF  HERMAPHRODITES    201 


Fig.  59.  —  Helix  arhustorum.  Diagram  showing  row  of  gorminal  cijitho- 
lial  cells  some  of  which,  as  at  m,  become  spermatogonia  and  droj) 
into  lumen  of  germ  gland;  others  beconu'  nurse  cells  {n)  ;  and  .still 
others  oocytes  {w).  Sgl  =  spermatogonium  of  first  order;  Sgll  = 
spermatogonium  of  second  order  ;  Sc  =  spermatocyte  ;  St  =  sperma- 
tid ;  Sp  =  spermatozoa.     {Fro7H  Buresch,  1011.) 


202        GERM-CELL   CYCLE   IN  ANIMALS 

gonia  of  the  second  order  (Fig.  58,  E).  Those  in- 
different germ  cells  that  are  to  form  oocytes  grow 
large,  remain  in  the  germinal  epithelium,  and  do  not 
divide.  They  possess  a  double  nucleolus  (Fig.  58,  F) . 
When  a  diameter  of  36  microns  is  attained,  the 
oocyte  passes  out  of  the  hermaphroditic  gland  into 
the  uterus. 

The  nurse  cells,  like  the  oocytes,  remain  in  the  wall 
and  do  not  divide ;  their  nuclei  grow  to  be  about  15 
microns  in  diameter  and  the  chromatin  forms  irregu- 
lar clumps  more  or  less  evenly  distributed  (Fig.  48,  B). 
No  differences  could  be  discovered  in  the  indifferent 
germ  cells  by  means  of  which  the  future  history  of 
these  cells  could  be  determined.  It  was  noted,  how- 
ever, that  egg  cells  were  never  present  without  a 
neighboring  nurse  cell,  and  the  conclusion  was 
reached  that  a  favorable  position  with  regard  to  a 
nurse  cell  determines  whether  an  indifferent  germ 
cell  shall  develop  into  a  spermatogonium  or  an  egg. 
If  Buresch's  observations  are  correct,  Helix  is  not 
protandric,  but  both  sorts  of  germ  cells  mature  at 
the  same  time,  and  the  fate  of  an  indifferent  germ 
cell  depends  upon  nutrition,  that  is,  its  proximity 
to  a  nurse  cell. 

Demoll  (19126)  has  proposed  a  new  hypothesis 
regarding  sex  determination  and  has  selected  certain 
events  in  the  oogenesis  and  spermatogenesis  of  Helix 
poinatia  as  arguments  in  its  favor.  The  hypothesis 
is  that  the  accessory  chromosome  (see  Chapter  IX) 
contains  the  anlagen  of  the  male  sexual  characters, 
whereas  the  female  sexual  characters  are  localized 


GERM  CELLS  OF  HERMAPHRODITES    203 

in  the  autosomes.  In  Helix  the  oogonia  and  sperma- 
togonia arise  from  cells  that  are  similar  in  size  and 
constitution  (Fig.  60,  A).  When  the  germ-cell 
nuclei  reach  the  bouquet  stage,  a  Nebenkern  appears 
near  the  side  against  which  the  chromatin  threads 


Fig.  60.  —  Helix  pomatia.  Stages  in  the  difforontiation  of  male  and 
female  sex  cells.  A.  Young  oocyte.  B.  Later  stage  of  oocyte 
showing  faint  Nebenkern.  C.  Young  spermatocyte.  D.  Later 
stage  of  spermatocyte  showing  well-marked  Nebenkern.  E.  Still 
later  stage  of  spermatocyte  containing  Nebenkern  consisting  of 
banana-shaped  rods.     (From  Demoll,  1912.) 

become  packed.  This  Nebenkern  is  probably  a 
product  of  the  nucleus;  it  appears  in  the  female 
cell  only  as  a  slightly  darker  area  of  cytoplasm  (Fig. 
60,  B)  but  in  the  male  cell  is  more  dense  (Z)),  later 
consisting  of  a  number  of  darkly  staining  banana- 
shaped  pieces  {E).  With  the  appearance  of  the 
Nebenkern  the  specific  growth  of  the  female   cells 


204        GERM-CELL   CYCLE   IN  ANIMALS 

is  initiated.  The  Nebenkern  disappears  in  the 
oocyte  soon  after  the  yolk  begins  to  form.  The 
chromatin  threads  in  the  spermatocytes  break  down 
and  lose  their  affinity  for  dyes,  but  later  reappear. 
In  the  oocyte,  on  the  contrary,  the  chromatin  threads 
persist.  Demoll  concludes  from  these  observations 
that  the  Nebenkern  always  determines  the  character 
of  the  germ  cells,  which,  up  to  its  formation,  may  be 
called  indifferent  germ  cells.  He  further  concludes, 
that,  since  in  dioecious  animals  sex  is  determined  by 
the  accessory  chromosomes,  in  Helix  the  sexual 
specificity  of  the  Nebenkern  must  be  determined 
by  the  accessory  chromosomes.  Such  chromosomes 
were  described  by  Demoll  (1912a)  in  a  previous 
contribution. 

A  similar  idea  has  been  expressed  by  von  Voss 
(1914)  regarding  the  differentiation  of  indifferent 
germ  cells  in  a  flat-worm,  Mesostoma  ehrenbergi. 
In  the  embryo  of  this  hermaphrodite  the  germ  gland 
is  a  syncytium  containing  both  the  nuclei  of  future 
oogonia  and  future  spermatogonia.  The  cytoplasm 
is  apparently  homogeneous  throughout.  The  forma- 
tion of  the  oogonia  from  indifferent  germ  cells  begins 
with  the  appearance  of  a  " germinal- vesicle  stage"; 
this  is  followed  by  an  increase  in  the  amount  of 
cytoplasm  surrounding  them.  Since  the  cytoplasm 
appears  to  be  similar  in  all  parts  of  the  syncytium, 
differentiation  must  be  initiated  by  the  nucleus, 
and  the  suggestion  is  made  that  perhaps  the  accessory 
chromosome  may  be  responsible  for  the  separation 
of  the  germ  cells  into  oogonia  and  spermatogonia. 


GERM   CELLS   OF   HERMAPHRODITES    205 

The  investigators  whose  results  have  been  de- 
scribed above  have  thus  furnished  three  theories  re- 
garding the  differentiation  of  male  and  female  germ 
cells  in  hermaphrodites :  (1)  In  Sagitta,  according 
to  Elpatiewsky,  it  is  an  unequal  distribution  of  the 
**besondere  Korper,"  (2)  in  Helix,  according  to  Ancel 
and  Buresch,  it  is  due  to  the  presence  or  absence  of  a 
nurse  cell  in  the  immediate  neighborhood,  and  (3)  in 
Helix,  Demoll  considers  it  a  result  of  the  influence  of 
the  accessory  chromosome.  It  is  perfectly  obvious 
that  hermaphrodites  offer  exceptionally  fine  material 
for  the  study  of  the  differentiation  of  germ  cells,  but 
that  thus  far  the  results  have  not  furnished  an  ade- 
quate explanation  of  the  phenomenon.  The  investi- 
gations of  Boveri  (1911),  Schleip  (1911),  and  Krueger 
(1912)  on  the  chromosomes  in  hermaphroditic  nema- 
todes may  be  discussed  more  profitably  during  the 
consideration  of  the  chromosome  cycle  in  the  next 
chapter. 

Certain  morphological  and  experimental  studies 
on  the  germ  glands  of  amphibians  are  of  interest  be- 
cause both  oogonia  and  spermatogonia  are  sometimes 
more  or  less  closely  associated  in  a  single  individual 
during  the  developmental  stages,  and  may  persist 
even  in  the  adult  germ  glands  of  a  number  of  species 
which  are  commonly  considered  dioecious.  Pfluger, 
for  example,  was  able  to  separate  the  young  of  the 
frog,  Rana  temporaria,  into  three  groups,  males,  fe- 
males, and  hermaphrodites ;  the  hermaphrodites 
developed  into  either  males  or  females.  Similar 
results   were    obtained    by  Schmidt -Marcel    (1908) 


206        GERM-CELL   CYCLE   IN  ANIMALS 

and  Kuschakewitsch  (1910),  who  refer  to  the  her- 
maphroditic individuals  as  intermediates. 

There  is  no  consensus  of  opinion  regarding  the 
origin  of  the  germ  cells  in  amphibians ;  one  group 
of  investigators,  including  Allen  (1907)  and  King 
(1908),  recognize  a  definite  keimbahn,  whereas  many 
others  (Semon,  1891;  Bouin,  1900;  Dustin,  1907; 
Kuschakewitsch,  1910 ;  Champy,  1913)  believe  they 
arise  from  the  germinal  epithelium  or  near-by  cells. 
Very  few  students  have  attempted  to  determine  the 
stages  in  or  causes  of  the  differentiation  of  male  and 
female  cells  from  the  primordial  germ  cells.  Kuscha- 
kewitsch (1910)  concludes  from  his  extensive  studies 
on  the  history  of  the  germ  cells  in  frogs  that  at  first 
all  of  the  germ  cells  are  indifferent  but  subsequently 
become  differentiated  in  two  directions.  Champy 
(1913)  has  studied  this  differentiation  in  a  number 
of  amphibians  and  has  concluded  that  if  the  charac- 
teristically lobed  or  polymorphic  nuclei  of  the  pri- 
mordial germ  cells  in  Bufo,  Hyla,  and  Rana  temporaria 
lose  their  original  shape  and  become  spherical  and 
clear,  the  germ  gland  will  form  an  ovary ;  but  if  the 
nuclei  retain  their  primitive  condition,  a  testis  will 
result.  Champy  believes  with  Kuschakewitsch  that 
both  sorts  of  germ  cells  arise  from  sexually  indifferent 
cells,  that  is,  sex  is  not  irrevocably  fixed  in  the  fer- 
tilized egg.  Furthermore  Champy' s  observations 
have  led  to  the  conclusion  that  the  germ  cells  in  the 
sexually  indifferent  germ  glands  are  morphologically 
identical  with  primitive  spermatogonia.  These  in- 
different germ  cells  become  differentiated  into  ova 


GERM   CELLS   OF  HERMAPHRODITES    207 

or  spermatozoa  as  a  result  of  various  causes,  some 
general  and  others  local  in  nature,  which  probably 
are  most  influential  at  certain  definite  stages  in  the 
cellular  activity.  A  new  equilibrium  is  thereby  es- 
tablished between  the  different  cell  organs  which 
initiates  new  processes  resulting  in  differentiation. 
The  undifferentiated  cells  in  the  testis  of  the  adult 
appear  also  to  be  identical  with  the  primitive  sper- 
matogonia, and  have  still  the  power  of  producing 
either  ova  or  spermatozoa.  Thus  the  male  amphib- 
ians are  also  females  **en  puissance,"  but  the  re- 
verse is  not  true.  This  accounts  for  the  numerous 
discoveries  of  ova  in  the  testes  of  these  animals. 

Reports  of  so-called  hermaphroditism  in  amphib- 
ians are  abundant  in  the  literature.  Cases  have 
been  reported  in  frogs  by  Cole  (1895),  Friedmann 
(1898),  Gerhartz  (1905),  Ognew  (1906),  Yung 
(1907),  Schmidt-Marcel  (1908),  Youngman  (1910), 
Hooker  (1912),  and  many  others.  Hooker  has  re- 
viewed the  literature  of  the  subject.  Hermaphrodit- 
ism in  other  amphibians  is  more  rare,  but  it  has 
been  noted  in  salamanders  by  La  Vallett  St.  George 
(1895)  and  Feistmantel  (1902).  Usually  the  condi- 
tion spoken  of  as  hermaphroditism  consists  in  the 
presence  of  ova  in  the  testis,  and  it  is  probable  that 
true  hermaphroditism  is  rare  in  these  animals  as  it 
is  in  other  vertebrates.  In  the  toad,  however,  a 
condition  exists  which  is  of  particular  interest.  The 
genital  ridge  of  every  toad  tadpole  15  to  18  days  old 
becomes  visibly  differentiated  into  two  regions,  an  an- 
terior portion  which  develops  into  Bidder's  Organ,  and 


208        GERM-CELL   CYCLE   IN   ANIMALS 

a  posterior  region  which  becomes  an  ovary  or  testis. 
Bidder's  Organ  persists  in  the  adult  of  males,  where 
it  lies  just  anterior  to  the  testis,  but  in  the  females 
of  Bufo  variabilis,  B.  cinereus,  B.  clamita,  and  B. 
lentiginosus  it  disappears  at  the  end  of  the  second 
year.  Bu^o  vulgaris  seems  to  differ  from  the  other 
species  since  here  Bidder's  Organ  persists,  becom- 
ing small  and  shrunken  during  the  winter  (Ognew, 
1906)  and  regenerating  during  the  summer  months 
(Knappe,  1886).  At  first  the  cells  in  both  the 
anterior  and  posterior  portions  of  the  genital  ridge 
are  similar,  all  possessing  a  polymorphic  nucleus, 
and  dividing  mitotically,  but  later  those  of  Bidder's 
Organ  begin  to  divide  amitotically  and  assume  the 
characteristics  of  young  oocytes  with  rounded  nuclei. 
Knappe  (1886)  claims  that  these  cells  never  become 
functional  ova  because  they  are  unable  to  form  yolk. 
King  (1908),  however,  does  not  consider  this  prob- 
able, but  traces  their  differentiation  to  irregularities 
in  the  synizesis  stage. 

By  most  investigators  Bidder's  Organ  is  regarded 
as  a  rudimentary  ovary.  Others  believe  that  the 
Amphibia  were  derived  from  hermaphroditic  ances- 
tors and  that  in  the  male  it  is  a  rudimentary  ovary 
and  in  the  female  a  rudimentary  testis.  This  seems 
more  probable  than  Marshall's  suggestion  that  this 
organ  is  the  result  of  degenerative  processes  proceed- 
ing backward  from  the  anterior  end  of  the  genital 
ridge,  or  than  that  it  represents  the  remains  of  a 
sex  gland  possessed  by  the  larvae  of  ancestral  toads 
when  they  were  paedogenetic,  as  Axolotl  is  at  the 


GERM   CELLS  OF  HERMAPHRODITES    209 

present  time.  Champy  (1913)  has  found  that  the 
cells  of  Bidder's  Organ  in  Bufo  pantherina  pass 
through  stages  in  their  transformation  similar  to 
those  of  the  primitive  germ  cells  of  liana  esculenta 
which  become  ova,  and  is  incHned  to  the  view  that 
the  principal  difference  between  the  toad  and  the 
intermediate  type  of  young  frogs  lies  in  the  fact 
that  in  the  former  the  oviform  cells  are  localized  in 
Bidder's  Organ,  whereas  in  the  frog  they  are  scattered 
throughout  the  germ  gland. 

The  development  of  the  germ  glands  in  the  hag- 
fish,  Myxine  glutinosa,  resembles  that  in  the  toad 
in  many  respects.  Cunningham  (1886)  and  Nansen 
(1886)  considered  Myxine  to  be  a  protandric  her- 
maphrodite. Schreiner  (1904),  however,  was  able  to 
show  that  every  adult  is  functionally  male  or  female 
with  a  rudimentary  ovary  anteriorly  situated  and  a 
posterior,  mature  testis,  or  a  functional  ovary  ante- 
rior to  a  rudimentary  testis.  These  results  were  con- 
firmed by  Cole  (1905). 

Similar  conditions  have  been  found  by  Okkelberg 
(1914)  in  the  young  of  the  brook  lampre3%  E?iio- 
sphenus  wilderi.  Of  fifty  larvae  ranging  from  7| 
cm.  to  20  cm.  in  length,  46  per  cent  were  true 
females,  10  per  cent  were  true  males,  and  44  per  cent 
were  hermaphrodites.  Since  male  and  female  adults 
are  approximately  equal  in  numbers,  it  was  concluded 
that  the  juvenile  hermaphrodites  become  adult 
males.  In  favor  of  this  conclusion  is  also  the  fact 
that  the  adult  males  frequently  possess  ova  in  their 
gonads  which  resemble  those  present  in  the  her- 
maphroditic larvae. 


210        GERM-CELL   CYCLE   IN   ANIMALS 

Regarding  the  differentiation  of  the  germ  cells  in 
hermaphrodites  then  we  may  recognize  two  principal 
views :  (1)  that  there  is  some  material  within  the 
cell  which  initiates  specialization,  or  (2)  that  differ- 
entiation is  due  to  general  or  local  causes  outside 
of  the  germ  cells.  The  former  is  favored  by  Elpatiew- 
sky  (1909,  1910)  from  studies  on  Sagitta  and  by 
Demoll  (1912)  from  studies  on  Helix.  The  second 
view  is  more  widely  advocated.  The  conclusions 
derived  by  Kuschakewitsch  (1910)  and  Champy 
(1913)  on  amphibians,  and  of  Ancel  (1903)  and 
Buresch  (1911)  on  Helix  agree  in  their  essential  fea- 
tures. All  of  these  investigators  maintain  that  the 
sex  cells  pass  through  an  indifferent  stage  and  are 
differentiated  into  oocytes  or  spermatocytes  because 
of  influences  external  to  themselves.  Buresch  and 
Champy  also  believe  that  even  in  the  fully  developed 
germ  glands  of  the  adult  these  primitive  cells  are 
present.  The  causes  of  their  differentiation,  how- 
ever, have  not  been  definitely  determined. 


CHAPTER  VIII 

KEIMBAHN-DETERMINANTS   AND    THEIR   SIG- 
NIFICANCE 

It  is  customary  to  be  suspicious  of  any  peculiar 
bodies  revealed  to  us  in  fixed  and  stained  material 
under  high  magnification.  There  can  be  no  doubt, 
however,  that  most,  if  not  all,  of  the  cytoplasmic 
inclusions  mentioned  in  the  preceding  chapters  are 
realities  and  not  artifacts.  Some  of  them  have  been 
seen  in  the  living  eggs ;  most  of  them  have  been  de- 
scribed by  several  investigators;  they  occur  after 
being  fixed  and  stained  in  many  different  solutions ; 
and  their  presence  is  perfectly  constant.  The 
genesis,  localization,  and  fate  of  these  bodies  are 
difficult  to  determine,  and  their  significance  is  prob- 
lematical ;  but  the  writer  has  attempted  in  the  follow- 
ing pages  to  draw  at  least  tentative  conclusions  from 
the  evidence  available  and  to  indicate  what  still 
needs  to  be  done. 

A.  The  Genesis  of  the  Keimbahn-determinants 

The  writers  who  have  discussed  the  origin  of  the 
keimbahn-determinants  have  derived  them  from 
many  different  sources.  In  a  few  cases  they  are  known 
to  be  nuclear  in  origin,  consisting  of  nucleolar  or  chro- 
matic materials;    they  are  considered  differentiated 

211 


212        GERM-CELL   CYCLE   IN   ANIMALS 

parts  of   the  cytoplasm   by  some  investigators;    in 
some  species  they  are  extra-cellular  bodies,  such  as 

nurse  cells. 

The  accompanying  table  indicates  the  number  and 
diversity  of  the  animals  in  which  keimbahn-determi- 
nants  have  been  described,  and  shows  the  increasing 
interest  that  has  been  given  to  this  subject  within  re- 
cent years,  over  half  of  the  papers  listed  having  been 
published  since  1908.  Several  cases  have  been  re- 
ferred to  in  the  text,  but  omitted  from  the  table  be- 
cause of  insuflacient  evidence  regarding  their  connec- 
tion with  the  primordial  germ  cells.  The  list  as 
given  includes  representatives  of  the  Ccelenterata, 
Ch^tognatha,  Nematoda,  Arthropoda,  and  Ver- 
TEBRATA.  The  tcrms  applied  to  the  various  sub- 
stances have  been  chosen  evidently  because  of  their 
genesis,  position  in  the  egg,  or  supposed  function. 

Table  of  Principal  Cases  of  Visible  Substances  con- 
cerned IN  Differentiation  of  Germ  Cells  (in  Chron- 
ological Order) 


Name  of  Species, 

Name  Applied  to 

AUTHOBITY 

Date 

Genus,  ob  Group 

Substance 

Chironomus  nigro- 

Dotterkornchen 

Weismann 

1863 

viridis 

Miastor 

Dottermasse 

Metchnikoff 

1866 

Moina  rectirostris 

Richtiingskorper 

Grobben 

1879 

Chironomus 

Keimwulst 

Ritter 

1890 

Daphnidae 

Paracopulations- 
zelle 

Weismann      and 
Ischikawa 

1889 

^quorea 

Metanucleolus 

Haecker 

1892 

Ascaris     megaloce- 

Chromatin 

Boveri 

1892 

phala 

KEIMBAHN-DETERMINANTS 


213 


A.  lumbricoides 

A.  niiiicunda 

Chromatin 

0.  Meyer 

189.5 

A.  labiata            ' 

Cyclops 

Aussenkornchen 

Haecker 

1897 

Ektosomen 

Haecker 

1903 

Calliphora 

Dotterplatte 

Noack 

1901 

Dytiscus 

Anello  cromatico 

Giardina 

1901 

Apis  mellifica 

Richtungskorper 

Petrunkewitsch 

190^2 

Parasitic               | 
Hymenoptera  J 

Nucleolo 

Silvestri 

1906 
1908 

Chrvsomelidae 

Pole-disc 

Hegner 

1908 

Miastor  metraloas 

polares  Plasma 

Kahle 

1908 

Sagitta 

besondere 
Korper 

Elpatiewsky 

1909 

Guinea-pig 

Chondriosom.es 

Rubaschkin 

1910 

Chick 

Chondriosomes 

Tschaschkin 

1910 

Lophius 

extruded 
plasmosome 

Dodds 

1910 

Ascaris 

Plasmadifferen- 

Boveri 

1910 

Chironomus 

zen 
Keimbahn- 
plasma 

Hasper 

1911 

Copepoda 

Ectosomen 

Amma 

1911 

Polyphemus 

Nahrzellenkern 

Kiihn 

fl911 
[1913 

Sagitta 

Keimbahn- 
chromidien 

Buchner 

1910 

Man 

Sertoli  cell 
determinant 

Montgomery 

1911 

Chick 

Attraction- 
sphere  ' 

Swift 

1914 

Parasitic 

Keimbahn- 

Hegner 

1914 

Hymenoptera 

chromatin 

a.  Nuclear.  Nucleoli.  It  seems  certain  that 
bodies  of  a  nucleolar  nature  behave  as  keimbahn- 
determinants.  There  are  three  or  more  kinds  of 
bodies  that  are  spoken  of  as  nucleoli.  Of  these  may 
be  mentioned  (1)  the  true  nucleoli  or  plasmosomes,  (2) 
karyosomes  or  chromatin-nucleoli,  and  (3)  double-nu- 


214        GERM-CELL  CYCLE  IN  ANIMALS 

cleoli,  consisting  of  usually  a  single  principal  nucleolus 
(Hauptnucleolus  of  Flemming),  and  one  or  more 
accessory  nucleoli  (Nebennucleoli  of  Flemming). 
Many  nucleoli  have  been  described  that  may  perhaps 
represent  intermediate  stages  in  the  evolution  of  one 
of  the  types  mentioned  above  into  another. 

The  young  ovarian  egg  of  most  animals  contains  a 
single  spherical  nucleolus  ("Keimfleck,"  or  "germi- 
nal spot"),  but  the  number  may  increase  greatly  dur- 
ing the  growth  period.  Usually  during  the  formation 
of  the  first  maturation  spindle  the  nucleolus  escapes 
from  the  nucleus  into  the  cytoplasm,  where  it  dis- 
appears, often  after  breaking  up  into  fragments. 
Many  theories  have  been  advanced  regarding  the 
origin,  function,  and  fate  of  the  nucleoli  of  the  germi- 
nal vesicle.  They  are  considered  by  some  of  chro- 
matic origin,  arising  as  an  accumulation  of  the  chro- 
matin, or  from  the  chromatin  by  chemical  trans- 
formation. Others  consider  them  extra-nuclear  in 
origin  (Montgomery,  1899). 

Many  functions  have  been  attributed  to  the  nu- 
cleoli ;  of  these  the  following  may  be  mentioned : 
(1)  They  function  as  excretory  organs  (Balbiani, 
1864 ;  Hodge,  1894) ;  (2)  nucleoli  play  an  active 
role  in  the  cell,  since  they  serve  as  storehouses  of 
material  which  is  contributed  to  the  formation  of  the 
chromosomes  (Flemming,  1882 ;  Lubosch,  1902 ; 
Jordan,  1910;  Foot  and  Strobell,  1912)  and  may 
give  rise  to  kinoplasm  (Strasburger,  1895)  or  *'Kine- 
tochromidien "  (Schaxel,  1910);  (3)  nucleoH  are 
passive   by-products   of    chromatic    activity;     they 


KEIMBAHN-DETERMIXANTS  215 

become  absorbed  by  active  substances  (Haecker, 
1895,  1899) ;  (4)  nucleoli  represent  nutritive  material 
used  by  the  nucleus  into  which  it  is  taken  from  the 
cytoplasm  (Montgomery,  1899). 

Undoubtedly  the  various  bodies  known  as  nucleoli 
originate  in  different  ways,  have  different  histories, 
and  perform  different  functions. 

In  the  particular  cases  to  be  discussed  here  the 
nucleoli  are  not  temporary  structures,  as  is  usually 
true,  but  persist  for  a  comparatively  long  interval  after 
the  germinal  vesicle  breaks  down.  What  seemed  to 
be  the  most  important  and  convincing  evidence  of 
the  functioning  of  a  nucleolus  as  a  keimbahn-determi- 
nant  is  that  furnished  by  Silvestri  (1906,  1908)  in 
parasitic  Hymenoptera.  As  shown  in  Chapter  V, 
however,  the  "nucleolo"  of  Silvestri  is  really  not  a 
nucleolus  but  consists  of  chromatin. 

As  we  have  already  noted,  in  a  few  instances  the 
nucleolus  does  not  disappear  during  the  maturation 
divisions  but  persists  for  a  time  as  a  "metanucleolus" 
(see  p.  183).  These  metanucleoli  are  evidently  of 
a  different  nature  from  the  usual  type  and  are  hence 
saved  from  immediate  disintegration  in  the  cyto- 
plasm. The  localization  of  the  metanucleolus  in  the 
egg  is  the  result  of  either  its  own  activity,  or  that  of 
the  surrounding  cytoplasm,  or  a  combination  of  these. 
Gravity  can  have  no  decided  effect  upon  it  (Herrick, 
1895),  since  its  position  is  constant,  whereas  the  posi- 
tion of  the  egg  with  respect  to  gravity  is  not.  It 
also  seems  hardly  possible  that  oxygenotactic  stimuli 
are  the  cause  of  its  localization,  as  has  been  suggested 


216        GERM-CELL   CYCLE   IN  ANIMALS 

by  Herbst  (1894,  1895)  for  the  migration  of  the 
blastoderm-forming  cells  from  the  center  to  the  sur- 
face of  the  eggs  of  certain  arthropods. 

Haecker  (1897)  has  suggested  that  the  "Aussen- 
kornchen"  which  appear  in  the  egg  of  Cyclops  during 
the  formation  of  the  first  cleavage  spindle  may  be 
nucleolar  in  nature.  Later  (1903)  this  idea  was 
withdrawn,  and  more  recently  Amma  (1911)  has 
likewise  been  unable  to  sustain  this  hypothesis.  The 
most  convincing  data  furnished  by  Amma  are  that  in 
an  allied  form,  Diaptomus  coeruleus  (Fig.  49,  H),  these 
granules  appear  before  the  cleavage  spindle  is  formed 
and  before  the  nucleoli  of  the  pronuclei  have  disap- 
peared. 

The  remaining  forms  in  which  nucleoli  have  been 
considered  as  keimbahn-determinants  are  merely 
suggestive.  In  Mquorea,  Haecker  (1892)  traced  the 
metanucleolus,  which  arises  from  the  germinal  vesicle, 
into  certain  cells  of  the  blastula.  Similar  bodies 
appear  in  Mitrocoma  (Metchnikoff,  1886),  Tiara 
(Boveri,  1890),  Stephanophyes  (Chun,  1891),  Myzo- 
stoma  (Wheeler,  1897),  and  Asterias  (Hartmann,  1902), 
but  their  ultimate  fate  has  not  been  determined. 
Meves  (1914),  however,  has  traced  the  middle  piece 
of  the  sperm  of  the  sea  urchin.  Par  echinus  miliaris, 
into  one  of  the  cells  of  the  animal  half  of  the  egg  at 
the  thirty-two-cell  stage.  This  middle  piece  is  of  a 
plastochondrial  nature. 

It  seems  probable  that  in  all  these  cases  the  same 
influences  may  be  at  work  regulating  the  time,  the 
place,  and  the  method  of  localization  of  the  nucleoli. 


keimbahn-deter:\il\ants         217 

The  writer  can  only  conclude  (1)  that  the  metanu- 
cleoli  differ  in  nature  from  ordinary  plasmosomes, 
chromatin-nucleoli,  and  double-nucleoli ;  (2)  that 
these  bodies  are  definitely  segregated  in  a  certain  part 
of  the  egg  or  in  a  certain  blastomere,  probably  by 
protoplasmic  movements ;  (3)  and  that  their  disin- 
tegration and  the  distribution  of  the  resulting  frag- 
ments or  granules  are  controlled  by  reactions  between 
them  and  the  substances  in  which  they  are  embedded. 
Chromatin.  In  two  genera  of  animals  the  differ- 
entiation of  the  primordial  germ  cells  is  accompanied 
by  a  diminution  of  the  chromatin  in  the  nuclei  of 
the  somatic  cells,  so  that  eventually  the  nucleus  of 
every  germ  cell  is  provided  with  the  full  complement 
of  chromatin,  whereas  the  nucleus  of  every  somatic 
cell  lacks  a  considerable  portion  of  this  substance, 
which  remains  behind  in  the  cytoplasm  when  the 
daughter  nuclei  are  reconstituted.  These  two  genera 
are  Ascaris  and  Miastor.  This  diminution  process 
was  described  by  Boveri  (1892)  in  the  former  and 
confirmed  by  O.  Meyer  (1895)  and  Bonnevie  (1902), 
and  by  Kahle  (1908)  in  Miastor  and  confirmed  by 
Hegner  (1912,  1914a).  For  details  of  these  processes 
reference  should  be  made  to  Figs.  15-16,  51-52,  and 
pp.  57  and  174.  It  may  be  pointed  out  here  that 
although  the  final  results  are  similar  the  process  dift'ers 
in  the  two  genera.  In  Ascaris  both  ends  of  each 
chromosome  are  split  off,  whereas  in  Miastor  approxi- 
mately one-half  of  each  daughter  chromosome  is  left 
behind  to  form  the  *'Chromosomenmitteli)hitte" 
(Fig.  16)  and  later  the  *'Chromatinreste"  (Fig.  18). 


218         GERM-CELL   CYCLE   IN  ANIMALS 

The  elimination  of  chromatin  during  the  matura- 
tion and  early  cleavage  divisions  of  the  egg,  as  well 
as  during  the  mitotic  divisions  of  other  kinds  of  cells, 
has  often  been  recorded.  For  example,  Wilson 
(1895,  p.  458)  estimates  that  only  about  one-tenth 
of  the  chromatin  in  the  germinal  vesicle  of  the  star- 
fish is  retained  to  form  the  chromosomes  during  the 
first  maturation  division,  and  Conklin  (1902)  finds 
that  "in  Crepidula  the  outflow  of  nuclear  material 
occurs  at  each  and  every  mitosis"  (p.  51).  Further- 
more, Rhode  (1911)  argues  that  chromatin-diminu- 
tion  is  a  normal  histological  process,  and  describes 
such  phenomena  in  blood  cells,  nerve  cells,  and 
cleavage  cells  of  several  Amphibia,  comparing  con- 
ditions with  the  chromatin-diminution  in  Ascaris 
and  Dysticus} 

Diminution  processes  similar  to  those  in  Ascaris 
and  Miastor  have  not  been  discovered  in  other  ani- 
mals, although  investigators  have  been  on  the  watch 
for  such  phenomena  and  have  studied  allied  species, 
e.g.,  the  work  of  Hasper  (1911)  on  Chironomus  and 
my  own  work  on  the  chrysomelid  beetles  (see  pp.  108 

^  His  conclusion  is  as  follows:  "In  der  Histogenese  der  allerver- 
schiedensten  Gewebe  tritt  uns  also  die  Erscheinung  entgegen,  dass 
die  sich  entwickelnden  Zellen,  bzw.  Kerne  einen  Teil  ihres  Chromatins 
abstossen,  d.  h.  also  eine  Chromatindiminution  erfolgt,  wenn  auch 
die  Befunde  selbst  im  speziellen  von  den  bisher  beobachteten  in  der 
Einleitung  beschriebenen  Fallen  der  Chromatindiminution  etwas  ab- 
weichen. 

"Eine  Chromatindiminution  tritt  also  nicht  nur  am  Anfang  und  Ende 
der  Keimbahn,  wie  es  bisher  angegeben  worden  ist,  sondern  in  den  ver- 
schiedensten  Entwicklungsstadien  und  bei  den  verschiedensten  Geweben 
undTieren  ein,  sie  hat  also  offenbar  eine  allgemeine  Bedeutung."    (p.  25.) 


KEIMBAHN-DETERMINANTS  219 

to  118).  If,  therefore,  there  is  a  similar  difference  in 
all  animals  in  chromatin  content  between  the  germ 
cells  and  somatic  cells,  the  elimination  of  chromatin 
from  the  latter  must  take  place  by  the  transformation 
of  the  basichromatin  of  the  chromosomes  into  oxy- 
chromatin  which  passes  into  the  cytoplasm  during 
mitosis,  or  else  by  the  more  direct  method  advocated 
by  the  believers  in  the  chromidia  hypothesis. 

The  causes  of  the  diminution  of  chromatin  in  As- 
caris  and  Miastor  are  unknown.  Recently  Boveri 
(1910)  has  concluded  from  certain  experiments  on 
the  eggs  of  Ascaris  (see  p.  177)  that  in  this  form  it  is 
the  cytoplasm  in  which  the  nuclei  are  embedded  that 
determines  whether  or  not  the  latter  shall  undergo 
this  process.  Kahle  (1908)  does  not  explain  the 
cause  of  the  diminution  in  Miastor.  To  the  writer  it 
seems  more  important  to  discover  why  the  nuclei 
of  the  keimbahn  cells  do  not  lose  part  of  their  chro- 
matin, since  the  elimination  of  chromatin  during 
mitosis  is  apparently  such  a  universal  phenomenon. 
I  would  attribute  this  failure  of  certain  cells  to  under- 
go the  diminution  process  not  to  the  contents  of  the 
nucleus  alone  but  to  the  reaction  between  the  nucleus 
and  the  surrounding  cytoplasm.  As  stated  in  a 
former  paper  (Hegner,  1909a),  *'In  Calligrapha  all 
the  nuclei  of  the  egg  are  apparently  alike,  potentially, 
until  in  their  migration  toward  the  surface  they 
reach  the  '  Keimhautblastem ' ;  then  those  which 
chance  to  encounter  the  granules  of  the  pole-disc 
are  differentiated  by  their  environment,  i.e.,  the 
granules,  into  germ  cells.     In  other  words,  whether  or 


220        GERM-CELL   CYCLE   IN   ANIMALS 

not  a  cell  will  become  a  germ  cell  depends  on  its  posi- 
tion in  the  egg  just  previous  to  the  formation  of  the 
blastoderm." 

Similarly  in  Ascaris  the  cleavage  nuclei  are  con- 
ceived as  similar  so  far  as  their  ''prospective  potency" 
is  concerned,  their  future  depending  upon  the  char- 
acter of  their  environment,  i.e.,  the  cytoplasm.  In 
the  egg  of  Miastor  cleavage  nucleus  IV  (Fig.  15)  does 
not  lose  part  of  its  chromatin  because  of  the  character 
of  the  reaction  between  it  and  the  substance  of  the 
"polares  Plasma."  In  chrysomelid  beetles  (Hegner, 
1908,  1909,  1914a)  and  Ckironomus  (Hasper,  1911), 
however,  although  no  diminution  process  has  been 
discovered  in  the  nuclei  that  encounter  the  pole-disc 
or  "Keimbahnplasma,"  the  other  nuclei  in  the  egg, 
so  far  as  known,  are  similar  in  this  respect.  The 
nuclei  of  the  primordial  germ  cells,  however,  may  be 
distinguished  easily  from  those  of  the  blastoderm 
cells  in  chrysomelid  beetles,  proving  conclusively 
that  a  differentiation  has  taken  place  either  in  one 
or  the  other.  This  differentiation  probably  occurs  in 
the  nuclei  that  take  part  in  the  formation  of  the 
blastoderm,  since  the  nuclei  of  the  germ  cells  retain 
more  nearly  the  characteristic  features  of  the  pre- 
blastodermic  nuclei,  w^hereas  those  of  the  blastoderm 
cells  change  considerably. 

In  some  cases  the  eliminated  chromatin  may  have 
some  influence  upon  the  histological  differentia- 
tion of  the  cell,  since  it  is  differentially  distributed 
to  the  daughter  cells,  but  in  Ascaris  and  Miastor 
no  mechanism  exists  for  regulating  the  distribution 


KEIMBAHN-DETERMINANTS  221 

of  the  cast-out  chromatin,  and  there  is  consequently 
no  grounds  for  the  hypothesis  that  "in  Ascaris  those 
cells  which  become  body  cells  are  the  ones  that  in- 
clude the  cast-off  chromosome  ends  in  their  cyto- 
plasm, and  it  will  pro})a})ly  be  found  that  these 
ejected  chromosome  parts  engender  such  cytoplasmic 
differentiations  as  characterize  the  body  cells " 
(Montgomery,  1911,  p.  192). 

Chromidia.  To  several  of  the  bodies  listed  in 
the  table  on  page  88  as  keimbahn-determinants  has 
been  ascribed  an  origin  from  the  chromatin  of 
the  germinal  vesicle.  Many  cases  of  the  elimination 
of  chromatin  from  the  nuclei  of  growing  oocytes  are 
to  be  found  in  the  literature.  Blochmann  (1886)  dis- 
covered a  process  of  "budding"  in  the  oocytes  of 
Camponotus  ligniperda  resulting  in  the  formation 
of  "Nebenkerne."  These  appear  first  as  small 
vacuoles  lying  near  the  nucleus ;  later  they  contain 
small  staining  granules  and  acquire  a  membrane. 
The  "Nebenkerne"  grow  in  size  and  increase  in  num- 
ber, while  the  nucleus  of  the  oocyte  becomes  smaller. 
Stuhlmann  (1886)  described  a  similar  phenomenon 
in  about  a  dozen  different  species  of  Hymenoptera. 
The  oocyte  nucleus  in  all  species  examined  becomes 
localized  near  the  anterior  end ;  then  the  small 
nuclear-like  bodies  form  around  it  at  its  expense. 
The  time  of  their  i)roduction  varies  in  the  differenl 
species ;  in  some  they  appear  in  the  very  young 
eggs ;  in  others  not  until  a  much  later  stage  has  been 
reached.  Sometimes  they  fuse  to  form  a  large 
"Dotterkern"  lying  at  the  posterior  pole  of  the  egg; 


222        GERM-CELL   CYCLE   IN   ANIMALS 

or  they  may  remain  separate  and  later  become  scat- 
tered. Paulcke  (1900)  also  noted  nuclear-like  bodies 
near  the  oocyte  nucleus  of  the  queen  bee,  and  Mar- 
shall (1907)  has  likewise  found  them  in  Polistes 
pallipes.  In  this  species  the  nuclear-like  bodies 
form  a  single  layer  around  the  nucleus;  later  they 
come  to  lie  near  the  periphery  of  the  oocyte  and 
finally  disappear.  Loewenthal  (1888)  has  described 
what  appears  to  be  chromatin  in  the  cytoplasm  of 
the  egg  of  the  cat,  and  an  elimination  of  chromatin 
was  noted  by  van  Bambeke  (1893)  in  the  ovarian 
egg  of  ScorpcBTia  scrofa.  In  none  of  these  species, 
however,  have  keimbahn-determinants  been  dis- 
covered. 

According  to  Buchner  (1910)  the  "besondere 
Korper"  in  the  egg  of  Sagitta,  and  in  fact  keimbahn- 
determinants  in  most  other  animals  are  of  a  chromid- 
ial  nature,  representing  the  tropho-chromatin  de- 
manded by  the  binuclearity  hypothesis.  The  term 
chromidia  was  introduced  by  R.  Hertwig  in  1902  and 
apphed  to  certain  chromatin  strands  and  granules 
of  nuclear  origin  in  the  cytoplasm  of  AdinosphoBrium. 
Goldschmidt  (1904)  transferred  the  chromidia  hy- 
pothesis to  the  tissue  cells  of  Ascaris.  Since  then 
chromidia  have  been  described  in  the  cells  of  many 
animals,  including  both  somatic  and  germ  cells. 
Thus  far  the  group  of  zoologists  that  favor  the 
chromidia  idea  have  not  received  very  extensive 
backing,  but  the  fact  remains  that  chromatin 
particles  are  in  some  cases  cast  out  of  the  nuclei  in 
the  oocytes  of  certain  animals  and  continue  to  exist 


KEIMBAHN-DETERMINAXTS  223 

as  such  in  the  cytoplasm  for  a  considerable  period. 
It  is  also  possible  that,  as  Buchner  (1910)  maintains, 
the  keimbahn-determinants  may  be  in  reality  **Keim- 
bahnchromidien." 

This  view  was  suggested  by  the  writer  in  1909 
(p.  274)  to  account  for  the  origin  of  the  pole-disc 
granules  in  the  eggs  of  chrysomelid  beetles.  It  was 
thought  that  here  as  in  the  Hymenoptera  (Bloch- 
mann,  1886 ;  et  at.)  chromatin  granules  might  be 
cast  out  of  the  nuclei  of  the  oocytes,  and  that  these 
granules  might  gather  at  the  posterior  end  to  form 
the  pole-disc.  It  was  also  suggested  that  chromatin 
granules  from  the  nurse-cell  nuclei  might  make  their 
way  into  the  oocyte  and  later  become  the  granules  of 
the  pole-disc.  It  should  not  be  forgotten,  moreover, 
that  these  granules  stain  like  chromatin.  Finally, 
mention  should  be  made  of  the  "anello  cromatico" 
of  Giardina  (1901)  which  is  associated  with  the 
differentiation  of  the  oocytes  in  Dytiscus  (see  p.  1^20, 
Fig.  38),  and  the  keimbahn-chromatin  which  I  have 
recently  described  (Hegner,  19146)  in  the  eggs  of 
the  parasitic  hymenopteron,  Copidosoma  (p.  151, 
Figs.  46-47). 

Conclusion.  Certain  keimbahn-determinants 
may  consist  of  nucleolar  material  which  is  derived 
from  the  germinal  vesicle  and  persists  until  the 
primordial  germ  cells  are  established.  In  some  cases 
the  keimbahn  cells  are  characterized  by  the  posses- 
sion of  the  complete  amount  of  chromatin  in  con- 
trast to  the  somatic  cells  which  lose  a  part  of  this 
substance.     Since,  however,  the  chromatin-diminu- 


224        GERM- CELL   CYCLE   IN  ANIMALS 

tion  process  does  not  occur  in  many  species,  it  is 
probably  not  a  universal  phenomenon,  and  conse- 
quently cannot  be  of  fundamental  importance.  Most 
of  the  evidence,  on  the  other  hand,  points  toward 
the  conclusion  that  all  of  the  cleavage  nuclei  are 
qualitatively  alike,  and  that  the  cytoplasm  is  the 
controlling  factor. 

b.  Cytoplasmic  or  Extracellular  Nutritive 
Substances.  It  was  pointed  out  on  a  preceding 
page  (p.  101)  that  one  of  the  characteristics  used  to 
distinguish  primordial  germ  cells  from  other  embry- 
onic cells  is  the  presence  within  them  of  yolk  material. 
In  many  vertebrates  the  yolk  globules  persist  in  the 
primordial  germ  cells  until  a  comparatively  late 
stage,  and  indeed  are  often  so  numerous  as  to  practi- 
cally conceal  the  nuclei  of  these  cells.  A  large  num- 
ber of  the  keimbahn-determinants  that  have  been 
described  are  supposed  to  consist  of  nutritive  sub- 
stances. Some  of  the  earliest  investigators  were 
aware  of  the  yolk  content  of  the  primordial  germ 
cells.  For  example,  in  Chironomus  Weismann  (1863) 
found  four  oval  nuclei  lying  in  the  "  Keimhautblas- 
tem  "  at  the  posterior  end  of  the  egg,  each  of  which 
is  associated  with  one  or  two  yolk  granules ;  these 
are  the  "Polzellen."  In  another  Dipteron,  Simula 
sp.,  Metchnikoff  (1866)  records  four  or  five  pole- 
cells  which  possess  fine  yolk  granules  in  their  cell 
substance.  The  same  author  (1866)  also  states  that 
when  the  pseudovum  in  the  psedogenetic  larva  of 
Miastor  contains  twelve  to  fifteen  nuclei,  one  of 
these,  together  with  the  dark  yolk-mass  in  which  it 


KEIMBAHN-DETERMINANTS  225 

lies,  is  cut  off  as  a  cell  which  gives  rise  to  the  pole- 
cells. 

In  certain  Daphnid^,  Weismann  and  Ischikawa 
(1889)  describe  a  "Paracopulationszelle"  wliicli  is 
derived  from  the  contents  of  the  germinal  vesicle 
(seep.  163)  ;  but  the  recent  work  of  KUhn  (1911, 1913) 
renders  it  probable  that  this  body  is  nothing  but 
the  remains  of  a  nurse  cell.  The  "Dotterplatte" 
discovered  by  Noack  (1901)  at  the  posterior  end  of 
the  egg  of  Calliphora  (Fig.  34)  is  considered  by  this 
investigator  to  consist  of  yolk  elements.  In  previous 
communications  (Hegner,  1908,  1909,  1911)  the 
writer  has  discussed  the  probability  that  the  pole-disc 
in  chrysomelid  eggs  consists  of  nutritive  material, 
and  Weiman  (1910a)  also  has  offered  arguments 
for  this  view. 

The  granules  segregated  in  certain  cleavage  cells  of 
Neritina  (Blochmann,  1881),  Asplanchna  (Jennings, 
1896),  Lepas  (Bigelow,  1902),  Siphonaria  (Fujita, 
1904),  and  Physa  (Wierzejski,  1906)  may  be  of  a 
nutritive  nature,  and  these  cells  may  be  the  stem 
cells  from  which  the  germ  cells  of  these  animals 
eventually  arise.  The  hypothesis  that  the  nucleoli 
consist  of  food  substance  also  argues  in  favor  of  the 
idea  that  the  keimbahn-determinants  are  nutritive. 

The  importance  of  these  nutritive  substances 
to  the  primordial  germ  cells  can  be  stated  with  some 
degree  of  certainty.  According  to  some  authorities 
the  primordial  germ  cells  remain  in  the  j)rimitive 
condition  and  do  not  undergo  differentiation  at  the 
same  time,  or  at  least  at  the  same  rate,  as  do  the 


ne        GERM-CELL  CYCLE  IN  ANIMALS 

other  embryonic  cells.  On  this  account  their  yolk 
contents  are  not  at  first  utilized,  since  their  meta- 
bolic activities  are  so  slight.  This  is  more  especially 
true  of  the  vertebrates  in  which,  it  has  been  sug- 
gested (Hegner,  1909a,  p.  276),  the  yolk  contents 
of  the  germ  cells  are  transformed  into  the  energy  of 
motion  during  the  characteristic  migration  of  these 
cells  into  the  germinal  epithelium.  Why  these 
nutritive  substances  are  segregated  in  the  primordial 
germ  cells  is  more  difficult  to  answer.  Finally,  it  is 
interesting  to  note  that  the  differentiation  of  the 
indifferent  germ  cells  of  Helix  arbustorum  into  sper- 
matogonia or  oogonia  has  been  found  to  depend 
upon  nutrition  (Buresch,  1911).^ 

Yolk  Nucleus.  There  are  many  bodies  in  the 
cytoplasm  of  growing  oocytes  that  have  been  called 
yolk  nuclei  and  that  may  be  responsible  for  the 
origin  of  the  keimbahn-determinants.  Some  of 
these  bodies  have  already  been  considered,  but  the 
term  'y^lk  nucleus'  has  been  applied  to  so  many 
different  cytoplasmic  inclusions  (Munson,  1912) 
that  no  attempt  will  be  made  here  to  describe  them 
nor  to  trace  their  history. 

Mitochondria.  The  condition  of  the  chondrio- 
somes  in  the  primordial  germ  cells  of  certain  verte- 
brates (Rubaschkin,  1910,  1912;  Tschaschkin,  1910; 
Swift,  1914)  and  the  theories  that  have  been  pro- 

i"Ob  aber  eine  indifferente  Geschlechtszelle  sich  in  mannlicher  oder 
weiblicher  Richtung  weiter  entwickeln  wird,  das  konnen  wir  schon  sehr 
friih  sagen,  namlich  nach  der  Lage  dieser  Zelle  naher  oder  weiter  von 
einer  Nahrzelle  "  (p.  327). 


KEIMBAHN-DETERMINANTS  227 

posed  regarding  the  role  of  these  bodies  in  heredity 
make  it  necessary  to  refer  to  them  briefly  here.  At 
the  present  time  it  is  difficult  to  make  any  definite 
statement  regarding  the  origin,  nature,  and  signifi- 
cance of  the  various  cytoplasmic  inclusions  that  have 
been  grouped  under  the  general  title  of  mitochondria. 
It  seems  probable  that  we  are  concerned  with  a  num- 
ber of  different  sorts  of  inclusions,  and  with  various 
stages  in  their  evolution.  In  the  guinea  pig  (Ru- 
baschkin,  1910,  1912)  and  chick  (Tschaschkin,  1910) 
the  chondriosomes  of  the  cleavage  cells  are  spherical 
and  all  similar,  but,  as  development  proceeds,  those 
of  the  cells  which  become  differentiated  to  produce 
the  germ  layers  unite  to  form  chains  and  threads, 
whereas  those  of  the  primordial  germ  cells  remain 
in  a  spherical  and  therefore  primitive  condition 
(Fig.  31,  B).  Swift  (1914)  has  found,  however, 
that  in  the  chick  the  mitochondria  in  the  primordial 
germ  cells  are  not  at  all  characteristic,  resembling 
those  of  the  somatic  cells.  The  germ  cells  neverthe- 
less can  be  distinguished  from  the  latter  by  the  pres- 
ence of  an  especially  large  attraction-sphere  (Fig. 
31,  D).  This  distinction  between  the  primordial 
germ  cells  and  the  surrounding  somatic  cells  may 
enable  us  to  trace  the  keimbahn  in  vertebrates  back 
into  cleavage  stages  —  something  that  has  not  been 
accomplished  as  yet. 

An  examination  of  the  various  keimbahn-deter- 
minants  Hsted  in  the  table  (p.  212)  has  led  the  writer 
to  conclude  that  none  of  them  is  of  a  mitochondrial 
nature,  but  the  results  obtained  by  the  special  methods 


228        GERM-CELL  CYCLE   IN  ANIMALS 

employed  by  students  who  are  studying  mitochondria 
give  us  good  reason  to  hope  that  other  substances 
may  be  made  visible  which  will  help  to  clear  up  the 
problem  of  primary  cellular  differentiation. 

Metabolic  Products.  Among  the  most  difficult 
cases  to  explain  are  those  of  Sagitta  and  certain  cope- 
pods,  since  here  the  keimbahn-determinants  ap- 
parently arise  de  novo  in  the  cytoplasm.     Buchner's 

(1910)  contention  that  the  "besondere  Korper" 
of  Sagitta  is  the  remains  of  the  *' accessory  fertiliza- 
tion cell"  of  Stevens  (1904)  is  not  sustained  by  either 
Stevens  (1910)  or  Elpatiewsky  (1910).  The  idea  of 
the  nucleolar  nature  of  the  *'Aussenkornchen "  in 
Cyclops  has  been  discarded  by  Haecker  (1903) 
and  the  conclusion  reached  that  these  granules  are 
similar  to  nucleoli  in  one  respect,  namely,  they  are 
by-products  of    activities  within  the  cell.       Amma 

(1911)  has  considered  this  subject  at  some  length, 
and  after  rejecting  the  possibiHties  of  these  being 
of  (1)  chromatic,  (2)  nucleolar,  (3)  chromidial,  and 
(4)  mitochondrial  origin  likewise  concludes  that 
they  are  transitory  by-products.  In  this  way 
the  keimbahn-determinants  in  copepods  are  satis- 
factorily explained,  and  a  similar  explanation  may 
be  applied  to  Sagitta,  although  with  less  certainty. 

c.  Discussion.  A  review  of  the  literature  on  the 
keimbahn-determinants  and  the  investigation  of  these 
substances  in  the  eggs  of  insects  force  me  to  conclude 
that  the  fundamental  organization  of  the  egg  is  respon- 
sible for  the  segregation  of  the  primordial  germ  cells, 
whereas  the  visible  substances  simply  furnish  evi- 


KEIMBAHN-DETERMINANTS  229 

dence  of  this  underlying  organization.  As  I  have 
stated  elsewhere  (Hegner,  1908,  p.  21)  regarding  Ihe 
keimbahn-determinants  in  beetles'  eggs,  "the 
granules  of  the  pole-disc  are  therefore  either  the  germ- 
cell  determinants  or  the  visible  sign  of  the  germ-cell 
determinants."  The  writer's  experiments  have  thus 
far  failed  to  determine  the  exact  function  of  these 
granules.  When  the  posterior  end  of  a  freshly  laid 
beetle's  egg  is  pricked  with  a  needle,  not  only  the 
pole-disc  granules  flow  out,  but  also  the  cytoplasm 
in  which  they  are  embedded  (Hegner,  1908).  If  a 
small  region  at  the  posterior  end  is  killed  with  a  hot 
needle,  the  pole-disc  is  prevented  from  taking  part 
in  the  development  of  the  egg,  but  so  also  is  the  sur- 
rounding cytoplasm  (Fig.  37,  c).  Eggs  thus  treated 
continue  to  develop  and  produce  embryos  without 
germ  cells,  but  as  a  rule  a  part  of  the  posterior  end 
of  the  abdomen  is  also  absent  (Hegner,  1911a).  The 
pole-disc  granules  and  the  cytoplasm  containing 
them  is  moved  by  centrifugal  force  toward  the  heavy 
end  of  the  egg  and  is  proved  to  be  quite  rigid,  but 
eggs  thus  treated  do  not  develop  sufficiently  normally 
to  enable  one  to  decide  whether  the  pole-disc  pro- 
duces germ  cells  in  its  new  environment  or  not. 

That  the  germ  cells  of  Chironomus  arise  from  a  pre- 
locaHzed  substance  was  stated  by  Balbiani  (1885)  in 
these  words,  "the  genital  glands  of  the  two  sexes 
have  an  absolutely  identical  origin,  arising  from 
the  same  substance  and  at  the  same  region  of 
the  egg''  Ritter  (1890)  expressed  the  opinion 
that  the  "Keimwulst"  of  Chironomus  consists  of  fine 


230        GERM-CELL  CYCLE   IN  ANIMALS 

granulated  protoplasm,  an  opinion  concurred  in  by 
Hasper  (1911),  who  terms  it  "Keimbahnplasma." 
The  similar  material  in  Miastor  metraloas,  the 
*'polares  Plasma,"  is  considered  a  special  sort  of 
protoplasm  by  Kahle  (1908),  and  I  can  confirm  this 
for  Miastor  americana.  Further  evidence  of  the 
protoplasmic  nature  of  the  substances  which  be- 
come segregated  in  the  primordial  germ  cells  is  fur- 
nished by  Boveri's  experiments  on  Ascaris.  In 
1904  this  investigator  concluded  from  a  study  of 
dispermic  Qggs  that  the  diminution  process  is  con- 
trolled by  the  cytoplasm  and  not  by  an  intrinsic  prop- 
erty of  the  chromosomes,  and  that  the  chromosomes 
of  nuclei  lying  in  the  vegetative  cytoplasm  remain 
intact,  whereas  those  of  nuclei  embedded  in  the 
animal  cytoplasm  undergo  diminution.  This  con- 
clusion has  been  strengthened  by  more  recent  experi- 
mental evidence  (Boveri,  1910)  both  from  observa- 
tion on  the  development  of  dispermic  eggs  and 
from  a  study  of  centrifuged  eggs  (see  p.  178,  Fig. 
53).  Boveri's  results  furnish  a  remarkable  confirma- 
tion of  the  conclusions  reached  by  the  writer  from  a 
morphological  study  of  the  germ  cells  of  chrysomelid 
beetles  and  expressed  in  the  following  words:  "All 
the  cleavage  nuclei  in  the  eggs  of  the  above-named 
beetles  {Calligrapha  multipunctata,  etc.)  are  poten- 
tially alike  until  in  their  migration  toward  the  periph- 
ery they  reach  the  'keimhautblastem.'  Then  those 
which  chance  to  encounter  the  granules  of  the  pole- 
disc  are  differentiated  by  their  environment,  i.e.,  the 
granules,  into   germ   cells;    all   the   other   cleavage 


KEIMBAHN- DETERMINANTS  231 

products  become  somatic  cells."  Here,  however, 
the  pole-disc  granules  were  considered  the  essential 
substance. 

The  appearance  of  the  keimbahn-determinants  at  a 
certain  time  and  in  a  certain  place,  and  their  deter- 
minate segregation,  point  unmistakably  to  an  under- 
lying regulating  mechanism.  These  phenomena  have 
some  definite  relation  to  the  fundamental  organiza- 
tion of  the  egg  and  require  an  investigation  of  our 
present  knowledge  of  this  subject. 

The  isotropism  of  the  egg  as  postulated  by  Pfluger 
and  the  "cell  interaction"  idea  especially  developed 
by  O.  Hertwig  and  Driesch  have  given  way  before 
the  beautiful  researches  tending  to  uphold  the  hy- 
pothesis of  "germinal  localization"  proposed  by  His 
and  championed  by  so  many  investigators  within 
the  past  two  decades.  The  starting  point  for  embry- 
ological  studies  has  shifted  from  the  germ  layers 
to  the  cleavage  cells  and  from  these  to  the  undivided 
egg.  Organization,  which  Whitman  (1893)  main- 
tains precedes  cell-formation  and  regulates  it,  is  now 
traced  back  to  very  early  stages  in  the  germ-cell 
cycle  and  held  responsible  for  the  cytoplasmic  lo- 
calization in  the  egg. 

One  of  the  fundamental  characteristics  of  the  egg  is 
its  polarity.  It  has  been  known  for  about  thirty 
years  that  the  eggs  of  insects  are  definitely  ori- 
ented within  the  ovaries  of  the  adults.  Moreover, 
gravity  and  the  action  of  centrifugal  force  have  no 
effect  upon  the  polarity  of  insect  eggs  (Hegner,  1909^) . 
Giardina  (1901)  has  found  that  during  the  divisions 


232        GERM-CELL  CYCLE  IN  ANIMALS 

of  the  oogonia  in  Dytiscus  a  rosette  of  sixteen  cells 
is  produced  of  which  one  is  the  oocyte  and  the  other 
fifteen  nurse  cells.  The  rosette  thus  formed  possesses 
a  definite  polarity  coincident  with  the  axis  of  the 
oocyte  which  is  identical  with  that  which  was  present 
in  the  last  generation  of  oogonia.  Similarly  in 
Miastor  (Fig.  12)  the  polarity  of  the  oocyte  is  recog- 
nizable as  soon  as  the  mesodermal  cells,  which  serve  in 
this  species  as  nurse  cells,  become  associated  with  it. 

The  germ  cells  of  other  animals  also  possess  a 
precocious  polarity,  as  evidenced  by  their  implanta- 
tion in  the  germinal  epithelium  {e.g.,  Wilson,  1903 ; 
Zeleny,  1904,  in  Cerebratulus),  the  position  of  the 
nucleus,  the  formation  of  the  micropyle  (Jenkinson, 
1911),  etc.  This  is  true  not  only  for  the  inverte- 
brates, but,  as  Bartelmez  (1912)  claims,  "the  polar 
axis  persists  unmodified  from  generation  to  genera- 
tion in  the  vertebrates  and  is  one  of  the  fundamental 
features  of  the  organization  of  the  protoplasm"  (p. 
310).  Furthermore,  experiments  with  centrifugal 
force  seem  to  prove  that  the  chief  axis  of  the  egg  is  not 
altered  when  substances  are  shifted  about,  but  is 
fixed  at  all  stages  (Lillie,  1909;  Morgan,  1909; 
Conklin,  1910).  Bilaterality  also  is  demonstrable 
in  the  early  stages  of  the  germ  cells  of  many  animals, 
and,  like  polarity,  seems  to  be  a  fundamental  charac- 
teristic of  the  protoplasm. 

It  is  somewhat  difficult  to  harmonize  the  various 
results  that  have  been  obtained,  especially  by  experi- 
mental methods,  from  the  study  of  egg  organization. 
As  the  oocytes  grow,  the  apparently  homogeneous 


KEIMBAHN-DETERMINANTS  233 

contents  become  visibly  different  in  some  animals, 
and  when  the  mature  eggs  develop  normally  these 
"organ-forming  substances"  are  segregated  in  def- 
inite cleavage  cells  and  finally  become  associated 
with  definite  organs  of  the  larva. 

Conklin  (1905)  has  shown  "that  at  least  five  of 
the  substances  which  are  present  in  the  egg  (of 
Cynthia)  at  the  close  of  the  first  cleavage,  viz., 
ectoplasm,  endoplasm,  myoplasm,  chymoplasm, 
and  chordaneuroplasm,  are  organ-forming  sub- 
stances." Under  experimental  conditions  "they 
develop,  if  they  develop  at  all,  into  the  organs  which 
they  would  normally  produce;  and,  conversely, 
embryos  which  lack  these  substances,  lack  also  the 
organs  which  would  form  from  them."  "Three  of 
these  substances  are  clearly  distinguishable  in  the 
ovarian  egg  and  I  do  not  doubt  that  even  at  this 
stage  they  are  differentiated  for  particular  ends" 
(p.  220).  "The  development  of  ascidians  is  a  mosaic 
work  because  there  are  definitely  localized  organ- 
forming  substances  in  the  egg;  in  fact,  the  mosaic 
is  one  of  organ-forming  substances  rather  than  of 
cleavage  cells.  The  study  of  ctenophores,  nemer- 
tines,  annelids,  mollusks,  ascidians,  and  amphibians 
(the  frog)  shows  that  the  same  is  probably  true  of  all 
these  forms  and  it  suggests  that  the  mosaic  principle 
may  apply  to  all  animals"  (p.  221).  The  same 
writer  has  also  proved  from  his  study  on  Phallusia 
(1911)  that  these  various  substances  exist  even  when 
they  are  not  visible  in  the  living  egg.  It  is  interesting 
also  to  note  that  Duesberg  (1913)  finds  the  "niyo- 


234        GERM-CELL  CYCLE   IN  ANIMALS 

plasm"  of  Cynthia  to  be  crowded  with  plasmosomes, 
differing  in  this  respect  from  other  egg  regions. 

Experiments,  especially  those  of  Lillie  (1906,  1909), 
Morgan  and  Spooner  (1909),  Morgan  (1909a),  and 
Conklin  (1910),  have  shown  that  in  many  eggs  the 
shifting  of  the  supposed  organ-forming  substances 
has  no  influence  upon  development,  and  leads  to  the 
conclusion  that  these  visible  substances  play  no 
fundamental  role  in  differentiation,  but  that  the 
invisible  ground  substance  is  responsible  for  de- 
terminate development.  The  eggs  of  different  ani- 
mals, however,  differ  both  in  time  and  degree  of 
organization,  and  the  conflicting  results  may  be 
accounted  for  by  the  fact  that  specification  is  more 
precocious  in  some  than  in  others. 

The  most  plausible  conclusions  from  a  considera- 
tion of  these  observations  and  experiments  are  that 
every  one  of  the  eggs  in  which  keimbahn-determi- 
nants  have  been  described  consists  essentially  of  a 
fundamental  ground  substance  which  determines 
the  orientation ;  that  the  time  of  appearance  of 
keimbahn-determinants  depends  upon  the  preco- 
ciousness  of  the  egg ;  that  the  keimbahn-determi- 
nants are  the  visible  evidences  of  differentiation  in 
the  cytoplasm ;  and  that  these  differentiated  portions 
of  the  cytoplasm  are  definitely  localized  by  cytoplas- 
mic movements,  especially  at  about  the  time  of 
maturation. 


KEIMBAHN-DETERMIXAXTS  235 

B.   The  Localization   of  the   Keimbahn-deter- 

MINANTS 

One  of  the  characteristics  of  the  keinibahn- 
determinants  is  their  regular  appearance  at  a  certain 
stage  in  the  germ-cell  cycle  according  to  the  species  in 
which  they  occur,  and  their  constant  localization 
in  a  definite  part  of  the  egg,  or  in  one  or  more  definite 
cleavage  cells.  Keimbahn-determinants  are  recog- 
nizable in  many  insects'  eggs  before  fertilization  is 
accomplished,  and  even  before  the  oocyte  has  reached 
its  maximum  size.  We  know  that  in  Chironomus 
the  ''Keimwulst"  (Ritter,  1890)  or  ''Keimbahn- 
plasma"  (Hasper,  1911)  is  present  when  the  egg 
is  laid,  at  which  time  the  pronuclei  as  a  rule  have 
not  yet  fused.  This  is  true  also  of  the  "Dotter- 
platte"  in  CaUiphora  (Noack,  1901).  There  can 
be  little  doubt,  however,  that  these  substances 
are  present  as  such  in  the  eggs  before  fertilization, 
judging  from  our  knowledge  of  the  history  of  similar 
materials  in  the  eggs  of  other  insects.  The  "pole- 
disc"  in  the  eggs  of  chrysomelid  beetles  (Hegner, 
1908;  Wieman,  1910a)  and  the  *'polares  Plasma" 
in  Miastor  (Kahle,  1908;  Hegner,  1912,  19Ua)  are 
recognizable  some  time  before  fertilization  and  cannot 
therefore  arise  because  of  any  influence  exerted  by 
the  spermatozoon.  Moreover,  in  Miastor  the  eggs 
thus  far  examined  have  all  been  part  hen  ogenetic. 
In  parasitic  Hymenoptera  the  Keimbahn-chromatin 
appears  in  both  fertilized  and  parthenogenelic  eggs 
at  an  early  growth  period.     In  only  one  animal  not 


236         GERM-CELL   CYCLE   IN   ANIMALS 

an  insect  has  a  similar  occurrence  been  noted,  namely, 
in  Polyphemus,  where,  according  to  Kuhn  (1911, 
1913),  the  keimbahn-determinants  consist  of  the 
remains  of  one  or  more  nurse  cells  (Fig.  50).  In  the 
Daphnid^  (Weismann  and  Ischikawa,  1889)  the 
"Paracopulationszelle"  arises  from  material  cast  out 
by  the  germinal  vesicle ;  in  Mquora  (Haecker,  1892) 
the  *'Metanucleolus"  is  likewise  derived  from  the 
germinal  vesicle ;  in  Ascaris  (Boveri,  1892)  chroma- 
tin-diminution  occurs  during  the  two-  to  four-cell 
stage;  in  Cyclops  (Haecker,  1897,  1903)  and  other 
copepods  (Amma,  1911)  the  "  Aussenkornchen "  or 
*'Ectosomen"  become  visible  soon  after  fertiliza- 
tion (Diaptomus) ,  but  usually  not  until  the  pro- 
nuclei fuse  (other  species) ;  in  Sagitta  the  "be- 
sondere  Korper"  (Elpatiewsky,  1909,  1910)  or 
*'Keimbahnchromidien"  (Buchner,  1910)  appear  to 
arise  de  novo  after  fertilization,  although  if  Buchner's 
contention  that  they  are  the  remains  of  the  accessory 
fertilization  cells  is  correct,  they  should  be  classed 
with  the  "  Nahrzellenkern "  described  by  Kuhn 
(1911,  1913)  in  Polyphemus. 

It  is  thus  evident  that  the  keimbahn-determinants 
become  visible,  wherever  they  have  been  described, 
either  just  before  or  just  after  the  eggs  are  fertilized, 
or,  in  parthenogenetic  forms,  shortly  before  matura- 
tion and  cleavage  are  inaugurated. 

The  localization  of  the  keimbahn-determinants  at 
the  time  of  their  appearance  seems  to  be  predeter- 
mined. In  insects  the  posterior  end  of  the  egg  is 
invariably  the  place  where  these  bodies  occur.     In 


KEIMBAHN-DETERMIXAXTS  237 

species  whose  eggs  undergo  total  cleavage  they  are, 
under  normal  conditions,  segregated  in  one  definite 
blastomere  from  the  two-cell  stage  up  to  the  thirty- 
two-cell  stage,  as  a  rule,  and  are  then  distril)uted 
among  the  descendants  of  the  single  primordial 
germ  cell.  In  Ascaris  it  is  normally  the  cell  at  the 
posterior  (vegetative)  pole  that  fails  to  undergo 
the  diminution  process.  It  seems  therefore  that 
there  must  be  some  mechanism  in  the  Qgg  which 
definitely  localizes  the  keimbahn-determinants. 

The  segregation  of  these  substances  in  one  blas- 
tomere at  the  first  cleavage  division  is  a  result  of  their 
previous  localization,  but  in  later  cleavage  stages 
events  are  more  diflficult  to  interpret.  Both  Haecker 
(1897)  and  Amma  (1911)  have  attempted  to  explain 
the  distribution  of  the  "Ectosomen"  in  copepods  by 
postulating  a  dissimilar  infiuence  of  the  centrosomes 
resulting  in  the  segregation  of  these  granules  at  one 
end  of  the  mitotic  spindle  in  the  dividing  stem  cell. 
According  to  Zeigler's  hypothesis  the  centrosomes 
during  unequal  cell  divison  are  heterodynamic, 
and  Schonfeld  (1901)  believes  that  the  synizesis 
stage  is  due  to  the  attraction  of  the  chromosomes  by 
the  centrosomes.  It  is  well  known  that  in  many 
cases  where  unequal  cell  division  occurs  one  aster 
is  larger  than  the  other,  and  this  may  be  the  true 
interpretation  of  the  phenomena,  but  to  the  writer 
it  seems  more  probable  that  the  entire  cell  contents 
undergo  rearrangement  after  each  cell  division, 
possibly  under  the  influence  of  the  material  elab- 
orated within  the  nucleus  and  set  free  during  iiiito- 


238         GERM-CELL   CYCLE   IN  ANIMALS 

sis.     Elpatiewsky  (1909)  also  believes  in  the  unequal 
attractive  force  of  the  centrosomes  in  Sagitta} 

In  Ascaris,  certain  copepods,  Sagitta,  Polyphemus^ 
and  certain  Daphnid^  the  keimbahn-determinants 
are  segregated  in  one  cleavage  cell  until  about  the 
thirty-two-cell  stage,  but  their  substance  is  dis- 
tributed at  the  next  division  between  the  daughter 
cells.  The  insects  such  as  Chironomus,  Miastor,  and 
chrysomelid  beetles,  where,  on  account  of  the  super- 
ficial cleavage  the  keimbahn-determinants  are  not 
segregated  in  blastomeres,  the  primordial  germ 
cells  from  the  beginning  consist  almost  entirely 
of  the  keimbahn  material  or  this  material  plus 
the  matrix  in  which  it  is  embedded.  Hence  in 
these  cases  the  keimbahn-determinants  are  localized 
at  a  determined  point  during  each  cleavage  stage 
instead  of  being  carried  about  by  the  movements  of 
the  egg  contents  or  of  the  blastomeres,  but,  as  in 
the  eggs  that  undergo  total  cleavage,  the  determi- 
nants are  distributed  between  the  daughter  cells  as 

^"Nach  der  vierten  Teilung  kommt  der  besondere  Korper  in  den 
Wirkungskreis  eines  Zentrosomos,  namlich  desjenigen,  welcher  naher 
der  Polarfurche  liegt.  Fast  die  ganze  'Energie'  dieses  Zentrosomas  wird 
fiir  die  Ueberwindung  der  vis  inertiae  des  besonderen  Korpers  ver- 
braucht ;  dieser  wird  dera  Zentrosoma  genahert  und  umschliesst  es  wie 
mit  einer  Kappe,  so  dass  er  im  optischen  Durchschnitt  stets  Hufeisen  oder 
Sichelform  aufweist.  Infolge  davon  wird  die  wirkung  dieses  Zentroso- 
mas auf  das  Zellplasma  nur  sehr  schwach,  dieses  Zentrosoma  kann  nur 
einen  kleinen  Plasmateil  beherrschen,  und  die  resultierende  Zelle  wird 
viel  kleiner,  als  die  Schwesterzelle.  Diese  kleine  Zelle,  die  den  beson- 
deren Korper  bekommen  hat,  liegt  naher  zum  vegetativen  Poles,  als 
die  grossere  Schwesterzelle,  und  stellt  die  erste  Urgeschlechtszelle 
G(d^^^),  die  grossere  Schwesterzelle  die  erste  Urentodermzelle  E(d^^2) 
vor"   (p.  231). 


KEIMBAHN-DETERxMINANTS  r,i9 

soon  as  the  primordial  germ  cells  are  esta})lis}ie(l. 
The  reason  for  this  appears  to  be  that  localizations 
occur  in  holoblastic  eggs  at  each  cleavage  and  that 
not  until  the  thirty-two-cell  stage  or  thereabouts 
does  the  keimbahn  material  become  entirely  sep- 
arated from  other  organ-forming  substances  and 
segregated  in  a  single  cell.  When  this  point  is 
finally  reached,  this  keimbahn  material  must  neces- 
sarily become  divided  between  the  daughter  cells. 

In  practically  all  known  cases  the  daughter  cells 
of  the  primordial  germ  cells  are  equal  in  size  and  each 
receives  an  equal  portion  of  the  keimbahn-de- 
terminants  (Fig.  37,  B).  This  is  certainly  to  be 
expected  from  their  constitution  and  future  historv. 
Sagitta,  however,  differs  in  this  respect,  for  the  remains 
of  the  "besondere  Korper"  appear  to  be  unequally 
distributed  between  the  two  daughter  cells  of  the 
primordial  germ  cells  (Fig.  54)  and  both  Elpatiewsky 
(1909,  1910)  and  Stevens  (1910),  therefore,  consider 
this  as  probably  a  differential  division  whereby  in  this 
hermaphroditic  animal  the  substance  of  the  male 
primordial  germ  cell  is  separated  from  the  female. 
More  work  is  necessary  to  make  certain  of  this  point. 

Conclusion.  Keimbahn-determinants  are  def- 
initely localized  in  the  egg  and  in  definite  cleavage 
cells.  This  localization  is  first  observable  just 
before  or  just  after  the  eggs  are  fertilized,  or,  in 
parthenogenetic  forms,  shortly  before  maturation 
and  cleavage  are  inaugurated.  Some  mechanisui  in 
the  egg  must  be  responsible  for  this  localization. 
Heterodynamic  centrosomes  may  have  some  influence 


240        GERM-CELL  CYCLE   IN  ANIMALS 

so  far  as  the  segregation  of  the  keimbahn-determi- 
nants  in  cleavage  cells  is  concerned,  but  the  move- 
ment of  the  egg  contents  seems  to  be  a  more  probable 
cause  of  localization. 

C,   The  Fate  of  the  Keimbahn-determinants 

It  is  unfortunately  impossible  to  trace  the  keim- 
bahn-determinants throughout  the  entire  germ-cell 
cycle.  The  question  of  their  fate,  however,  is  an 
important  one.  As  we  have  seen,  they  become  vis- 
ibly apparent  shortly  before  or  just  after  the  inaugu- 
ration of  the  maturation  divisions,  and  remain  intact 
for  a  brief  period  during  the  early  cleavage  stages. 
They  persist  in  insects  as  definitely  recognizable 
granules  (Fig.  37,  F)  for  some  time  after  the  primor- 
dial germ  cells  are  segregated ;  then  they  gradually 
break  up  into  finer  particles,  leaving  no  trace  of  their 
existence  behind  except  in  so  far  as  they  give  the 
cytoplasm  of  the  germ  cells  a  greater  afiinity  for 
certain  dyes.  In  Chironomus  they  may  still  form 
distinct  masses  after  the  definitive  germ  glands 
have  been  formed  (Fig.  33,  D).  The  ectosomes  in 
the  copepods  are  temporary  bodies  which  appear 
to  rise  de  novo  during  the  formation  of  each  mitotic 
figure  in  the  early  cleavage  stages,  then  break  down 
and  disappear.  Practically  all  of  the  other  keim- 
bahn-determinants persist  during  early  cleavage  and 
then  disappear  as  distinct  visible  bodies  as  soon  as  the 
primordial  germ  cells  are  definitely  segregated.  What 
becomes  of  them  during  the  comparatively  long 
period  between  their  disappearance  in  the  primordial 


KEIMBAHN-DETERMINAXTS  ^241 

germ  cells  and  their  reappearance  in  the  oocytes  or 
mature  eggs  can  only  be  conjectured.  They  seem  to 
disintegrate  into  very  fine  particles  which  become 
thoroughly  scattered  within  the  cell  body  and  mixed 
with  the  cytoplasm.  It  has  been  suggested  (p.  68) 
that  they  may  retain  their  physiological  characteris- 
tics and  become  concentrated  again  in  the  growing 
oocytes  into  morphologically  similar  bodies,  in- 
creasing in  the  meantime,  by  multiplication  or  in 
some  other  way,  until  they  equal  in  mass  those  of 
the  preceding  generation  of  germ  cells.  On  the  other 
hand,  they  may  all,  like  the  ectosomes  of  copepods, 
be  temporary  structures  produced  at  a  certain  time 
and  place  under  similar  metabolic  conditions,  and, 
becoming  associated  with  particular  parts  of  the 
cell  contents,  thus  be  constant  in  their  distribution. 
Several  ideas  have  been  advanced  regarding  the 
fate  of  the  eliminated  chromatin  in  Ascaris.  The 
ends  of  the  chromosomes  which  are  cast  out  into  the 
cytoplasm  are  not  equally  distributed  among  the 
daughter  cells  nor  does  there  appear  to  be  any  mech- 
anism for  their  definite  unequal  division.  These 
facts  argue  against  the  theory  that  these  cast -out 
chromatin  bodies  serve  as  determinants  and  also 
make  improbable  the  hypothesis  that  they  enal)le 
the  somatic  cells  to  differentiate,  whereas  the  germ 
cells  which  do  not  undergo  the  diminution  process 
remain  in  an  indifferent  condition,  since  their  cyto- 
plasm lacks  this  material  (Montgomery,  1911,  p.  7\H). 
However,  the  fact  that  during  the  early  cleavage 
divisions  in  some  animals  (see  p.  218)  large  amounts 

R 


242        GERM-CELL   CYCLE   IN  ANIMALS 

of  chromatin  escape  from  the  nucleus  and  are  dif- 
ferentially distributed  to  the  daughter  cells  is  evidence 
that  nuclear  material  may  play  some  important  role 
in  the  progressive  changes  of  cleavage  cells. 

It  has  been  shown  that  in  many  animals  the  germ 
cells  do  not  multiply  for  a  considerable  period 
during  the  early  developmental  stages.  This  period 
coincides  also  with  that  during  which  the  keimbahn- 
determinants,  as  a  rule,  disappear.  For  example, 
the  germ  cells  of  chrysomelid  beetles  multiply  until 
there  are  about  sixty -four  present,  at  which  time  they 
constitute  a  group  at  the  posterior  end  of  the  egg  and 
the  embryo  has  just  started  to  form ;  no  further 
increase  in  number  occurs  until  the  larval  stage  is 
reached  and  the  definitive  germ  glands  are  established. 
As  soon,  however,  as  the  embryo  has  reached  a 
certain  developmental  stage,  the  germ  cells  migrate 
into  it,  and  it  looks  very  much  as  though  they  remain 
quiescent  until  the  somatic  cells  are  *'able  to  protect, 
nourish,  and  transport"  them. 

The  number  of  primordial  germ  cells  during  the 
**period  of  rest"  is  perhaps  most  definitely  known  in 
Miastor,  where,  as  one  group  of  eight  and  later  as  two 
groups  of  four  each,  they  are  present  throughout  a 
large  part  of  embryonic  development. 

In  vertebrates  also  a  long  period  exists  during 
which  division  of  the  primordial  germ  cells  does  not 
take  place  (Fig.  6)  and  at  least  in  several  species 
certain  cell  contents  (the  mitochondria)  remain  in  an 
indifferent  condition  (Rubaschkin,  1910;  Tschasch- 
kin,  1910 ;  Fig.  31,  B).     These  facts  all  indicate  that 


KEIMBAHN-DETERMIXANTS  243 

these  cells  remain  in  a  primitive  condition  and  do 
not  undergo  the  histological  differentiations  charac- 
teristic of  somatic  cells,  a  view  which,  however,  has 
been  objected  to  (Eigenmann,  189G).  The  disap- 
pearance of  the  keimbahn-determinants  and  the 
yolk  globules  of  vertebrates  during  this  period  have 
suggested  that  these  substances  are  nutritive  in 
function,  furnishing  energy  to  the  migrating  germ 
cells. 

The  fact  of  this  long  rest  period,  followed  by  rapid 
multiplication  of  the  oogonia  and  spermatogonia 
during  which  no  important  specializations  occur,  and 
later  succeeded  by  the  remarkable  changes  that  occur 
in  both  the  oocytes  and  spermatocytes,  has  led  to  the 
suggestion  (Montgomery,  1911,  pp.  790-792)  that  in 
the  germ-cell  cycle  there  is  a  series  of  changes 
parallel  with  that  of  the  somatic  cycle.  In  the 
development  of  both  cycles  preformation  and  epi- 
genesis  proceed  at  the  same  time.  The  chromosomes 
seem  to  be  the  preformed  elements  of  the  germ  cells, 
since  they  are  apparently  the  most  stable  constitu- 
ents. The  cytoplasm,  on  the  other  hand,  undergoes 
a  series  of  epigenetic  changes  such  as  the  formation 
of  an  idiozome,  the  development  of  mitochondria, 
the  appearance  of  a  sphere,  and  the  metamorphosis 
of  the  spermatozoon. 

Finally  we  must  inquire  into  the  fate  of  the  keim- 
bahn-determinants in  the  male  germ  cells.  Does  the 
keimbahn  material  in  these  cells  increase  in  amount  as 
has  been  suggested  for  the  oocytes  and  is  it  localized 
in  the  spermatogonia,  spermatocytes,  or  spernuitozoa 


244        GERM-CELL   CYCLE   IN  ANIMALS 

as  a  definite,  visible  substance  ?  We  know  from  the 
investigations  of  Meves  (1911)  that  the  plastosonies 
in  the  spermatozoon  are  carried  into  the  egg,  in  the 
case  of  Ascaris,  and  there  fuse  with  the  plastosomes 
of  the  ovum.  Whether  keimbahn-determinants  act 
in  a  similar  manner  is  unknown.  There  are,  how- 
ever, certain  cytoplasmic  inclusions  in  the  male 
germ  cells  that  have  been  compared  with  similar 
structures  in  the  oocytes,  for  example,  the  chromatic 
body  described  by  Buchner  (1909)  in  the  spermato- 
genesis of  Gryllus  (see  p.  88),  and  the  plasmosome 
which  is  cast  out  of  the  nucleus  of  the  second  sperma- 
togonia in  Periplatieta  and  disintegrates  in  the  cy- 
toplasm (Morse,  1909).  That  keimbahn-determi- 
nants from  the  spermatozoon  are  not  necessary  for 
the  normal  production  of  germ  cells  is  of  course  evi- 
dent, since  some  of  the  species  with  which  we  are 
best  acquainted,  for  example,  Miastor,  are  partheno- 
genetic. 


CHAPTER  IX 

THE     CHROMOSOMES    AND     MITOCHONDRIA     OF 

GERM    CELLS 

No  account  of  the  germ-cell  cycle  in  animals  can  be 
considered  complete  without  at  least  a  brief  reference 
to  the  history  of  the  chromosomes  and  mitochondria 
of  germ  cells.  The  chromosomes  have  for  many 
years  been  recognized  as  the  most  important  visible 
bodies  in  the  cell,  and  their  behavior  during  the  germ- 
cell  cycle  has  convinced  most  zoologists  that  they 
may  also  be  regarded  as  the  bearers  of  hereditary 
factors.  The  mitochondria,  on  the  other  hand, 
are  cellular  constituents  which  have  only  compara- 
tively recently  come  into  prominence  in  cytological 
literature,  and  ideas  concerning  their  nature  and 
functions  are  still  in  a  very  chaotic  condition. 

The  Chromosome  Cycle  in  Animals 

A  few  general  statements  regarding  the  behavior  of 
the  chromosomes  during  cell  division,  maturation, 
and  fertilization  are  contained  in  Chapters  I  nnd  II. 
We  may  recognize  a  rather  definite  chromosome  cyc-le 
as  a  part  of  the  germ-cell  cycle,  and  it  is  to  certain 
events  in  this  chromosome  cycle  that  our  attention 
will  be  directed  in  the  following  i)aragraphs.  It  is 
best  to  begin  our  discussion,  as  in  the  gencrnl  review 

245 


246        GERM-CELL  CYCLE   IN  ANIMALS 

of  the  germ-cell  cycle  (Chapter  II),  with  the  par- 
thenogenetic  or  fertilized  egg  after  the  maturation 
processes  have  been  completed,  and  to  exclude  all 
references  to  the  accessory  chromosome  until  later. 

It  may  be  pointed  out  first  that  the  number  of 
chromosomes  in  the  cells  of  any  individual  of  a 
species  is,  with  few  exceptions,  constant.  Thus  the 
thread  worm  of  the  horse,  Ascaris  megalocephala 
var.  Mnivalens,  has  two;  A.  megalocephala  var. 
hivalens,  four ;  the  nematod,  Coronilla,  eight ;  the 
mole  cricket,  Gryllotalpa  vulgaris,  twelve;  the 
bug,  Pentatoma,  fourteen ;  the  rat,  sixteen ;  the 
sea  urchin.  Echinus,  eighteen ;  the  salamander, 
Salamandra  maculosa,  twenty-four ;  the  slug,  Limax 
agrestis,  thirty-two ;  and  the  brine  shrimp,  Artemia, 
one  hundred  and  sixty-eight.  This  number,  however, 
is  reduced  one-half  during  the  maturation  of  the 
eggs  and  spermatozoa  so  that  the  mature  eggs  and 
spermatozoa  possess  only  half  as  many  chromosomes 
as  the  other  cells  in  the  body ;  for  example,  the  body 
cells,  oogonia,  and  spermatogonia  of  the  rat  are 
provided  each  with  sixteen  chromosomes,  but  the 
mature  eggs  and  spermatozoa  contain  only  eight. 
Parthenogenetic  eggs  differ  from  those  that  require 
fertilization,  since  in  these  the  complete  or  diploid 
number  of  chromosomes  is  retained.  When  cleavage 
is  inaugurated  in  such  eggs,  a  spindle  is  formed,  the 
chromosomes  are  halved,  and  each  daughter  cell 
acquires  one-half  of  each  chromosome  as  in  ordinary 
mitosis.  In  fertilized  eggs,  however,  the  nucleus 
brought  in  by  the  spermatozoon  fuses  more  or  less 


CHROMOSOMES  AND   MITOCHONDRIA     247 

completely  with  the  egg  nucleus  and   the  two  to- 
gether  become   incorporated   in   the   first   cleavage 


Fig.  61.  —  Independence  of  paternal  and  maternal  chromatin  in  the 
segmenting  eggs  of  Cyclops.  A.  First  cleavage-figure  in  ('.  .strcmtus; 
complete  independence  of  paternal  and  maternal  chromosonu-s. 
B.  Resulting  two-cell  stage  with  double  nuclei.  C.  Second  cleavage  ; 
chromosomes  still  in  double  groups.  D.  Blastomeres  with  double 
nuclei  from  the  eight-cell  stage  of  C.  brevicornis.  (From  Wilson, 
after  Haecker.) 

spindle.     Each  of  the  two  nuclei  furnishes  an  equal 
(haploid)     number    of    chromosomes    to    the    first 


248        GERM-CELL   CYCLE   IN  ANIMALS 

cleavage  spindle,  and  thus  the  diploid  (somatic) 
number  is  regained.  These  chromosomes  may  there- 
fore be  considered  as  forming  two  groups,  one  group 
of  paternal  origin  derived  from  the  sperm  nucleus, 
and  one  group  of  maternal  origin  derived  from  the 
egg  nucleus ;  in  fact  the  groups  supplied  by  the  two 
nuclei  may  remain  perfectly  distinct  (Fig.  61),  not 
only  during  the  first  cleavage  division,  but  also 
during  subsequent  mitoses. 

The  chromosomes  of  the  fertilized  egg  and  of 
the  cells  to  which  it  gives  rise  are  not  always  of  the 
same  size  and  shape,  but  in  many  cases  are  known  to 
differ  morphologically  from  one  another.  It  is 
possible  to  recognize  the  different  chromosomes 
during  each  mitosis,  and  the  evidence  is  quite  con- 
vincing that  morphologically  similar  pairs  are  present 
in  every  cell  and  that  one  member  of  each  pair  is 
derived  from  the  egg  nucleus,  the  other  from  the 
sperm  nucleus.  Two  principal  views  are  held  re- 
garding the  character  of  the  chromosome  divisions 
during  the  early  cleavage  divisions,  (1)  that  the 
chromatin  granules,  which  represent  definite  de- 
terminers, are  divided  equally  between  the  daughter 
chromosomes,  and  (2)  that  an  unequal  distribution 
of  the  granules  occurs,  thus  forming  daughter  cells 
containing  qualitatively  different  chromosomes. 
There  are  no  observations  which  show  an  unequal  dis- 
tribution. 

One  of  the  changes  that  takes  place  in  the  chromo- 
somes at  the  time  of  mitosis  is  the  diminution  of  their 
chromatin  content  brought  about  by  the  passage  of 


CHROMOSOMES   AND    MITOCIIOXDUIA     249 

part  of  their  substance  into  the  cytoplasm.  This 
phenomenon  has  been  used  as  an  argument  in 
favor  of  the  theory  of  nuclear  control  of  cellular 
activities.  Two  special  cases  of  chromatin-diniinu- 
tion  are  known  which  differ  from  the  usual  process ; 
these  occur  in  Ascaris  and  Miastor  as  described  and 
figured  in  Chapters  III  and  VI.  In  these  animals  a 
large  portion  of  the  chromosomes  of  certain  nuclei 
is  cast  out  into  the  cytoplasm,  w^hereas  all  of  the 
chromatin  is  retained  by  others;  the  latter  witli  a 
complete  amount  become  the  nuclei  of  the  germ  cells, 
the  rest  with  a  reduced  amount  are  present  in  all  of  the 
somatic  cells. 

During  the  cellular  divisions  which  result  in  the 
multiplication  of  the  somatic  cells  and  of  the  ])rimor- 
dial  germ  cells  the  chromosomes  appear  at  each 
mitosis  in  their  normal  number  and  are  apparently 
divided  equally  between  the  daughter  cells.  There 
are,  however,  certain  variations  in  both  the  somatic 
and  germinal  mitoses.  In  the  somatic  cells  only 
one-half  the  normal  number  may  appear;  thus  in 
the  snail,  Helix  pomatia,  the  number  may  be  twenty- 
four  instead  of  the  usual  forty-eight.  T]um'(^  is 
reason  to  believe  that  each  of  these  twenty-four  really 
consists  of  two  single  (univalent)  chromosomes, 
and  may  therefore  be  considered  bivalent.  Even 
a  further  reduction  in  number  by  the  association  of 
univalent  chromosomes  has  been  recorded,  in  which 
case  the  combined  chromosomes  are  said  to  be  pluri- 
valent.  Other  variations  in  the  number  of  chronio- 
somes,  which  occur  during  the  maturation  of  the 
germ  cells,  will  be  referred  to  later. 


250        GERM-CELL  CYCLE   IN  ANIMALS 

Certain  cellular  phenomena  which  concern  the 
chromosome  cycle  have  been  described  in  preceding 
chapters  and  so  need  only  be  mentioned  here.  First, 
the  occurrence  of  amitosis  in  the  multiplication  of 
the  germ  cells  has  an  intimate  relation  to  the  speci- 
ficity of  the  chromosomes,  since  if  nuclei  divide  en 
masse  it  seems  improbable  that  the  chromosomes  be- 
come equally  divided  between  the  daughter  nuclei 
(see  Chapter  V,  p.  133) ;  and  second,  the  formation  of 
nurse  cells  from  oogonia  may  be  accompanied,  as 
in  Dytiscus  (Chapter  V,  p.  120),  by  a  chromatin- 
diminution  process  which  may  be  regarded  as  a 
differentiation  of  mother  germ  cells  into  somatic 
cells  (nurse  cells)  and  oogonia,  a  differentiation  re- 
sembling the  segregation  of  the  primordial  germ 
cells  in  the  cleavage  stages  of  the  ^gg. 

The  most  striking  and  perhaps  the  most  important 
stages  in  the  chromosome  cycle  occur  during  the 
growth  and  maturation  periods  of  the  germ  cells. 
As  briefly  described  and  figured  in  Chapter  II, 
the  mitoses  which  occur  during  maturation  are 
meiotic,  since  the  mature  germ  cells  have  their  chro- 
mosome number  reduced  one-half.  The  events  in  this 
process  most  worthy  of  our  attention  are  those  which 
take  place  during  the  stages  known  as  synapsis 
and  reduction.  Wilson  (1912)  has  summed  up  the 
questions  that  remain  to  be  solved  in  the  following 
words:  "The  cytological  problem  of  synapsis  and 
reduction  involves  four  principal  questions,  as 
follows  :  (1)  Is  synapsis  a  fact  ?  Do  the  chromatin- 
elements   actually   conjugate   or   otherwise   become 


CHROMOSOMES  AND   MITOCHONDRIA    251 

associated  two  by  two  ?  (2)  Admitting  the  fact  of 
synapsis,  are  the  conjugating  elements  chromosomes, 
and  are  they  individually  identical  with  those  of 
the  last  diploid  or  pre-meiotic  division  ?  (3)  Do 
they  conjugate  side  by  side  (parasynapsis,  parasyn- 
desis),  end  to  end  (telosynapsis,  metasyndesis), 
or  in  both  ways  ?  (4)  Does  synapsis  lead  to  partial 
or  complete  fusion  of  the  conjugating  elements  to 
form  'zygosomes'  or  'mixochromosomes,'  or  are 
they  subsequently  disjoined  by  a  'reduction-divi- 
sion '  ?  Upon  these  questions  depends  our  answer 
to  a  fifth  and  still  more  important  question,  namely, 
(5)  Can  the  Mendelian  segregation  of  unit-factors 
be  explained  by  the  phenomena  of  synapsis  and 
reduction  ?" 

The  behavior  of  the  chromosomes  during  synapsis 
in  the  germ  cells  of  the  male  is  indicated  diagram- 
matically  in  Fig.  62,  the  terms  used  being  those 
proposed  by  von  Winiwarter  (1901)  in  his  work  on 
the  oogenesis  of  the  rabbit.  In  the  spermatogonia 
(Fig.  62,  1)  the  chromatin  is  arranged  in  clumps  on 
an  achromatic  reticulum ;  in  the  spermatocyte 
(Fig.  62,  2)  it  breaks  up  into  granules  which  become 
arranged  in  single  rows  or  filaments  (the  leptotene 
threads).  These  leptotene  threads  later  become 
paired  (synaptene  stage.  Fig.  62,  3)  and  converge 
toward  the  side  of  the  nucleus  near  which  the  centro- 
some  and  centrosphere  are  situated  (Fig.  62,  4)»  a 
condition  known  as  synizesis.  The  granules  of  the 
leptotene  filaments  approach  and  finally  fuse  so 
as  to  produce  single   thick   threads  (Fig.  62,  5-7) ; 


252 


GERM-CELL  CYCLE  IN  ANIMALS 


this  is  the  pachytene  stage.     The  filaments  then 
begin  to  unravel  (Fig.  62,  6-7),  become  distributed 


c 


Fig.  62.  —  Prophases  of  the  heterotype  division  in  the  male  Axolotl. 
1,  nucleus  of  sperm ogonium,  or  young  spermocyte ;  2,  early  lepto- 
tene  ;  3,  transition  to  synaptene;  4,  synaptene  with  the  double 
filaments  converging  towards  the  centrosome  ;  5,  contraction  fig- 
ure; 6,  7,  pachytene;  8,  early;  9,  later  diplotene  ;  10,  the  hetero- 
typic double  chromosomes;  the  nuclear  membrane  is  disappearing. 
(Frorti  Jenkinson,  1913.) 

throughout  the  nucleus,  and  finally  split  into  two 
threads  (Fig.  62,  8-9) ;  this  is  the  diplotene  stage. 
The  pairs  of  filaments  finally  shorten  and  thicken, 


CHROMOSOMES   AND   MITOCHONDRIA    253 

assuming  the  form  of  paired  chromosomes  of  various 
shapes  and  sizes  (Fig.  62,  10).  A  spindle  then 
forms;  these  *' heterotypic"  chromosomes  are 
drawn  upon  it ;  and  each  daughter  cell  receives  one 
chromosome  of  each  pair. 

This  mitosis  is  called  heterotypic  because  it  differs 
from  ordinary  indirect  nuclear  division  in  two  im- 
portant respects :  (1)  the  chromosomes  are  ])resent 
in  pairs,  and  entire  chromosomes  are  separated,  and 
(2)  the  result  is  a  reduction  of  chromosomes  in  the 
daughter  nuclei  to  one-half  the  somatic  number. 
According  to  certain  investigators  {e.g.,  jNIeves, 
1907)  the  union  of  the  leptotene  threads  in  the  synap- 
tene  stage  (Fig.  62,  ^)  does  not  occur,  but  the  two 
parallel  threads  are  simply  the  halves  of  a  single 
longitudinally  split  filament  which  fuse  in  the 
pachytene  stage  (Fig.  62,  6-7),  and  separate  again 
in  the  diplotene  stage  (Fig.  62,  8-9).  The  large 
majority  of  cytologists,  however,  believe  that  the 
leptotene  threads  represent  chromosomes  which 
actually  fuse  in  pairs  in  the  pachytene  stage  and  sep- 
arate from  each  other  during  the  heterotypic  mitosis. 
Furthermore,  the  chromosomes  of  each  pair  are  con- 
sidered to  be  homologous,  that  is,  the  one  derived 
from  the  spermatozoon  is  morphologically  similar,  to 
its  mate,  which  is  derived  from  the  egg  nucleus. 

Investigators  who  believe  synapsis  to  be  a  fact, 
that  the  conjugating  elements  are  chromosomes, 
and  these  chromosomes  are  identical  with  those  of 
the  last  diploid  mitosis  are  not  agreed  as  to  the 
method  of  union  and  subsequent  separation  of  the 


254        GERM-CELL   CYCLE   IN  ANIMALS 

chromosomes.  The  chromosom'es  may  unite  side  by 
side  in  parasynapsis  or  end  to  end  in  telosynapsis. 
Apparently  parasynapsis  is  the  rule,  although  telosyn- 
apsis probably  occurs  in  certain  species.  The  results 
are  the  same  in  either  case. 

The  next  question  to  be  considered  is  whether  the 
chromosomes  which  emerge  from  the  pachytene  stage 
are  the  same  as  those  that  enter  it  as  leptotene 
filaments,  or  whether  there  is  a  complete  fusion 
into  zygosomes  or  mixochromosomes.  It  seems 
probable  that  at  least  a  partial  fusion  occurs 
and  that  the  composition  of  the  chromosomes  is 
changed  more  or  less  during  synapsis.  We  know 
for  certain  that  the  peculiar  X-chromosomes  which 
have  been  found  in  many  species  of  animals  become 
paired  in  synapsis  and  later  separate  in  a  true 
reduction  division,  and  we  also  have  evidence 
which  furnishes  a  mechanical  means  of  effecting 
a  change  in  the  chromosomes  during  the  synaptene 
stage.  This  evidence  has  led  to  the  formulation  of 
the  chiasmatype  theory  (Janssens,  1909).  Accord- 
ing to  this  theory  the  chromosomes  which  pair  in 
synapsis  may  twist  around  each  other  more  or  less 
(Fig.  63),  and  cross  connections  are  visible.  When 
the  paired  chromosomes  later  split  apart  they  rep- 
resent combinations  different  from  those  present 
before  synapsis,  because  of  these  cross  connections. 
The  results  of  experimental  breeding  seem  to  necessi- 
tate some  such  relation  as  this  during  synapsis,  and 
the  chiasmatype  theory  has  been  used  to  explain 
certain  results  of  hybridization  that  have  not  been 


CHROMOSOMES  AND   MITO(  HOXDRIA    255 

accounted   for   in   any   other   way    (Morgan,    1013, 
1914). 

The  view  that  the  chromosomes  are  the  ])earers  of 
factors  in  heredity  is  based  upon  several  liypotheses, 
of  which  those  of  their  specificity  and  genetic  con- 
tinuity will  be  mentioned  here.  According  to  the 
hypothesis  of  chromosome  specificity  each  chromo- 
some possesses  certain  functions  of  a  specific  kind 


A 

Fig.  63.  —  Twisting  of  chromosomes  according  to  the  chiasmat>T)e 
theory.  A.  Two  twisted  chromosomes  each  divided  longitudinally 
into  two.  B.  Twisted  chromosomes  of  Batracoseps  attenuatus. 
(From  Janssens,  1909.) 

which  determine  the  character  of  cellular  differen- 
tiation and  thus  the  structural  and  physiological 
condition  of  the  embryo,  larva,  and  adult.  The  hy- 
pothesis of  the  genetic  continuity  was  evolved  from 
that  of  the  individuality  of  the  chromosomes.  Ac- 
cording to  the  latter  theory  the  chromosomes  that 
appear  in  mitosis  do  not  become  scattered  during  the 
resting  stage  of  the  nucleus  (interkinesis),  but  re- 
tain their  identity  throughout  this  period.  Lack 
of  evidence  has  resulted  in  the  substitution  of  the 
hypothesis  of  genetic  continuity,  according  to 
which  there  is  a  definite  relation  between  the  chro- 
mosomes of  successive  mitotic  divisions. 

Much  of  the  cytological  literature  of  the  past  dec- 
ade deals  with  the  history  of  the  X-  or  sex-chromo- 


25Q        GERM-CELL   CYCLE   IN  ANIIVMLS 

somes.  For  many  years  the  number  of  chromosomes  in 
the  cells  of  the  individuals  of  a  species  was  considered 
constant  and  even.  Henking,  however,  in  1891, 
discovered  in  the  bug,  Pyrrhocoris,  sl  single  chro- 
mosome which  did  not  divide  in  one  of  the  sperma- 
tocyte divisions,  but  passed  to  one  of  the  daughter 
cells  and  hence  into  only  one-half  of  the  spermatozoa. 
Paulmier  (1899)  observed  similar  conditions  in 
the  squash  bug,  and  since  then  one  or  more  odd 
chromosomes  have  been  discovered  in  a  large  number 
of  animals  belonging  to  many  different  phyla.  In 
1902,  McClung  suggested  that  these  pecuHar  chro- 
mosomes might  be  sex-determinants,  and  subsequent 
discoveries  have  fully  demonstrated  that  they  are 
intimately  associated  with  the  phenomena  of  sex. 
Most  of  our  knowledge  of  this  subject  is  due  to  the 
investigations  of  cytologists  in  this  country,  es- 
pecially Montgomery  (1898,  1906,  1911),  McClung 
(1899,  1902,  1905),  Stevens  (1905,  1906,  1910), 
Wilson  (1905,  1906,  1911,  1912),  and  Morgan  (1909, 
1911,  1913,  1914).  A  few  of  the  principal  types  of 
sex-chromosome  distribution  are  as  follows : 

Type  I.  One  X-chromosome.  This,  the  simplest 
type,  has  been  recently  demonstrated  in  a  remarkable 
fashion  by  Mulsow  (1913)  in  a  nematoid  worm, 
Ancyr acanthus.  Here  the  chromosomes  can  be  seen 
not  only  in  stained  material  but  also  in  the  living 
germ  cells.  The  diploid  number  of  chromosomes  in 
male  worms  is  eleven  (Fig.  64,  A),  in  female  worms, 
twelve  {E) .  Two  sorts  of  spermatozoa  are  produced, 
one-half  with  five  and  the  other  half  with  six  chromo- 


Fig.  64.  —  Behavior  of  chromosomes  during  inaturatioii,  fertiliziition. 
and  cleavage  of  Ancyr acanthus  cystidicola.  {From  Mulsoir,  IIJIS.) 
A.  Spermatogonium  with  eleven  chromosomes.  B.  First  matura- 
tion (spermatocyte)  division.  The  single  chromosome  finally  joins 
one  group.  C.  The  four  spermatids  arising  from  one  spcrniatooyte  ; 
two  with  six  chromosomes,  and  two  with  five.  D.  Two  sperma- 
tozoa drawn  while  alive ;  one  with  six  chromosomes,  and  one  with 
five.  E.  Oogonium  with  twelve  chromosomes.  F.  Second  matura- 
tion (oocyte)  division.  The  black  mass  above  is  the  first  polar 
body;  the  set  of  six  black  chromosomes  are  those  of  the  second 
polar  body;  the  six  dotted  chromosomes  are  those  of  the  egg. 
G.  Fertilized  (male  producing)  egg ;  sperm  nucleus  above  with  five 
chromosomes;  egg  nucleus  below  with  six  chromosomes.  //.  fertil- 
ized (female  producing)  egg;  both  egg  and  sperm  nuclei  with  .six 
chromosomes.  /.  Cleavage  stage  of  male  producing  egg  :  the  central 
cell   with   nucleus    containing    eleven    chromosomes.     J.    Cleavage 


Prot^nor  <? 


Pro/eno/ 


t'€ 


Fig.  65.  —  Maturation  in  Pro^enor.  Male  above.  A.  Spermatogonium. 
B.  Synapsis.  C.  First  maturation  division.  Z),  D'.  Second  mat- 
uration division.     E,  E' .  Two  sorts  of  spermatozoa. 

FemaU  below.  A.  Oogonium.  B.  Synapsis.  C.  First  matura- 
tion division.  D.  Second  maturation  division.  E.  Egg  nucleus 
and  two  polar  bodies  all  alike  in  chromosome  content.  First  polar 
body  is  dividing.  {From  Morgan's  Heredity  and  Sex,  published  by 
the  Columbia  University  Press.)  (258) 


CHROMOSOMES   AND   MITOCHONDRIA     259 

somes  (Fig.  64,  D).  The  nuclei  of  all  the  inalure 
eggs  exhibit  six  chromosomes.  When  fertilized 
the  spermatozoon  nucleus  can  be  recognized,  since 
it  lies  near  the  end  away  from  the  polar  bodies.  On 
the  average  one-half  of  the  eggs  are  fertilized  by 
spermatozoa  containing  five  chromosomes  and  one- 
half  by  spermatozoa  containing  six.  The  results 
are  as  follows :  A  zygote  resulting  from  the  fusion 
of  an  egg  with  six  chromosomes  and  a  spermatozoon 
with  six  chromosomes  possesses  twelve  chromosomes 
and  develops  into  a  female  (Fig.  64,  //) ;  and  a 
zygote  formed  by  an  egg  with  six  chromosomes  and 
a  spermatozoon  with  five  chromosomes  contains 
eleven  chromosomes,  and  hence  gives  rise  to  a  nuile 
(Fig.  ,64,  G).  The  events  during  the  maturation 
processes  in  such  a  case  are  similar  to  those  in  the 
bug  Protenor,  as  illustrated  in  Fig.  Q5. 

Type  II.  One  X-chromosome  and  one  Y-chro- 
mosome.  In  the  bug,  Lygoeus  bicrucis,  and  a  num- 
ber of  other  species  the  number  of  chromosomes  in 
both  male  and  female  is  the  same,  but  two  sex-chro- 
mosomes of  different  sizes  are  present  in  the  male. 
As  shown  in  Fig.  66,  the  eggs  are  all  alike,  contain- 
ing six  ordinary  and  one  X-chromosome.  The  sper- 
matozoa are  of  two  sorts:  one-half  with  the  larger, 
or  X-chromosome,  the  other  one-half  with  the  smaller, 
called  by  Wilson  the  Y-chromosome.  The  zygotes, 
consequently,  produce  males  if  one  X-chromosome 
and  one  Y-chromosome  are  present,  and  fenudes 
if  two  X-chromosomes  occur. 

Type     III.     Two     chromosomes     of     equal     size 


260 


GERM-CELL   CYCLE   IN  ANIMALS 


i-yp' 


<a.eits 


A 


•*:?% 


•  ••• 

C 


Ly^aetts"  ^ 


•• 


»!*^ 


Fig.  66.  —  Maturation  in  Lygoeus.  Male  above.  Female  below.  Let- 
tering as  in  Fig.  65.  (From  Morgan  s  Heredity  and  Sex,  published 
by  the  Columbia  University  Press.) 


CHROMOSOMES  AND   MITOCHONDRIA     2^1 

(Fig.  67).  In  the  bug,  Oncopelius  fa.sriatus,  the 
number  of  chromosomes  (10)  in  both  male  and 
female  is  the  same,  but  they  are  of  ecjual  size  in 
both  sexes.  It  is  probable,  however,  that  one  of 
those  of  the  male  represents  an  X-chromosome  and 
the  other  a  Y-chromosome  as  in  Type  II,  although 
they  are  not  visibly  different. 

Type  IV.  One  X-chromosome  attached  to  an 
ordinary  chromosome.  There  are  a  number  of 
cases  on  record  in  which  the  X-chromosome  is 
attached  to  an  ordinary  chromosome  as  in  Ascaris 
megalocephala.  Probably  on  this  account  the  sex- 
chromosome  was  overlooked  in  these  species  for 
many  years.  The  resulting  zygotes,  as  Fig.  08 
show^s,  are  comparable  to  those  of  Type  I  (Fig.  65). 

Type  V.  Spermatozoa  alike,  but  eggs  of  two  sorts. 
In  a  few  animals  it  has  been  found  that  the  eggs  are 
dimorphic  and  the  spermatozoa  all  alike,  as  repre- 
sented in  Fig.  69.^ 

Numerous  variations  have  been  discovered  in 
the  number  and  size  of  the  X-  and  Y-chromosomes ; 
some  of  these  are  illustrated  in  Fig.  70.  When  more 
than  one  X-chromosome  is  present  they  act  as  a  unit, 
and  two  sorts  of  zygotes  are  produced  as  in  other  cases. 

Chromosome  cycles  of  more  than  ordinary  interest 
have  been  described  in  the  honeybee,  in  phyloxerans 
and  aphids  and  in  certain  hermaphrodites.  It 
has  long  been  known  that  the  female  honeybees 
(queens  and  workers)   develop  from    fertilized  eggs 

1  The  recent  contributions  of  Tennent  iind  Baltzcr  make  the  occurrence 
of  this  type  seem  very  doubtful. 


262 


GERM-CELL  CYCLE   IN  ANIMALS 


OnccpettoAf  3 


to*.* 


*.o°. 


fl  .^^ 


c 


••• 


••• 


w^ 


Wlii//- 


Onccpettu^  9 


JT' 


Fig.  67. —  Maturation  in  Oncopeltus.  Male  above.  Female  below. 
Lettering  as  in  Fig.  65.  (From  Morgans  Heredity  and  Sex,  pub- 
lished by  the  Columbia  University  Press.) 


CHROMOSOMES   AND    MITOCHONDRIA     263 


Escorts    cT 


«\} 


fik 


c 


^ 


r- 


2> 


\\ 


If 


© 

IB 


© 


^sootri^  9 


//I 


A 


#« 


Fig.  68.  — Maturation  in  .4scans.  Male  al)Ove.  Female  below.  Let- 
tering as  in  Fig.  05.  {From  Moruan's  Heredity  and  Sex,  publmfud 
by  the  Columbia  University  Press.) 


264 


GERM-CELL   CYCLE   IN  ANIMALS 


Reduction 
Division 


Ripe  Eggs 

Q»i  P'^^f^r  Body 


Oogonium 


^\y 


1//     < 


Spermat- 
ogonium 


'-/V 


s^/^ 


/w 


Fertilized 

Egg 


Fertilized 


Egg 


Fig.  69.  —  Diagrams  showing  the  behavior  of  the  chromosomes  during 
maturation  and  fertilization  in  the  starfish,  Echinus.  One  kind  of 
spermatozoon  is  formed,  but  the  ripe  eggs  differ,  one  containing  a 
large  X-element,  the  other  a  small  Y-element.     (From  Schleip,  1913.) 


X 


T 


om 


Protenor,  A^iasa     Syromastes,  Homo     Ascaris  lumhricoides 


Nazara 
viridula 


0 


V 

Euschisfus 
coemis 


6 


Nazara 
hilaris 


i    i 


0 


Thy ant  a 
calceata 


Rocconoia,      Prionides,      Gelasfocoris     Acholla 
Fitchia  Sinea  multispinosa 

Fig.  70.  —  Diagram  showing  the  number  and  size  relations  of  the  X-  and 
Y-chromosomes  in  a  number  of  animals.     {From  Wilson,  1911.) 


CHROMOSOMES   AND   MITOCHONDUIA     ^2G5 

and  the  drones  parthenogenetically.  The  liistory  of 
the  chromosomes  has  here  been  worked  out  by 
Nachtsheim  (1913).  The  primary  oocyte  contains 
sixteen  chromosomes  in  the  form  of  eight  tetrads  ;  the 
mature  egg  and  polar  bodies  are  each  provided  witli 
eight  chromosomes  (Fig.  71,  E)  ;  the  inner  half  of  the 
divided  first  polar  body  fuses  with  the  second  polar 
body,  forming  a  *'Richtungskopulationskern"  (Fig. 
71,  F)  which  does  not  give  rise  to  the  male  germ  cells 
as  Petrunkewitsch  (1901)  claimed,  but  degenerates. 

The  cleavage  nucleus  in  the  parthenogenetic  egg 
which  produces  the  male  shows  sixteen  chromosomes 
which  divide  to  form  thirty-two  or  sixty-four  in 
the  somatic  cells,  but  do  not  increase  in  number  in 
the  spermatogonia.  The  first  maturation  division 
is  unequal,  and  a  "polar  body"  without  any  chroma- 
tin is  pinched  off  (Fig.  71,  A-C,  Rh).  The  sperma- 
tids are  likewise  of  two  sorts;  the  smaller  (Fig.  71, 
C,  Rk2)  contain  as  many  chromosomes  as  the  larger 
(16),  but  degenerate,  while  the  larger  transform  into 
spermatozoa.  The  fertilized  (female)  eggs  possess 
the  same  number  of  chromosomes  as  the  partheno- 
genetic eggs,  plus  an  equal  number  which  is  brought 
in  by  the  spermatozoon.  The  cleavage  nucleus 
exhibits  thirty-two  chromosomes  wliich  may  become 
sixty-four  in  the  somatic  cells,  Init  unite  two  by 
two  to  form  sixteen  in  the  oogonia. 

Phylloxera  caryoBcaulis  will  serve  to  illustrate 
the  chromosome  cycle  in  a  species  with  a  life  cycle 
composed  of  parthenogenetic  females  which  alter- 
nate with  sexual  males  and  females   (Morgan,  1909, 


266 


GERM-CELL  CYCLE  IN  ANIMALS 


1910).     The  eggs  laid  by  the  stem-mother  (see  Chap- 
ter I,  p.  24)  in  the  spring  possess  four  ordinary  and 

B  C 


••  •. 


m^ 


;*^A??!- 


D  >-^- 


Fig.  71.  —  Stages  in  the  spermatogenesis  and  oogenesis  of  the  honeybee. 
A,  B.  First  maturation  division  in  the  miale.  C.  Second  matura- 
tion division  in  the  male.  Three  cells  are  produced  :  the  first  (RKi) 
without  chromatin ;  the  second  {RK-i)  with  chromatin,  but  small 
and  functionless ;  and  the  third  a  functional  spermatid.  {After 
Meves,  1907.) 

D.  First  maturation  division  in  the  female  showing  polar  body 
with  eight  dyads,  and  secondary  oocyte  with  eight  dyads.  E.  Sec- 
ond maturation  division  in  the  female  showing  the  divided  first 
polar  body,  the  second  polar  body,  and  female  pronucleus  each  with 
eight  monads.  F.  Outer  end  of  first  polar  body  disintegrating ; 
inner  half  of  first  polar  body  uniting  with  second  polar  body,  and 
female  pronucleus.     {After  Nachsheim,  1913.) 

two   sex    chromosomes.     These    eggs   give    rise   to 
parthenogenetic  females  with  the  same  number  of 


CHROMOSOMES  AND   MITOCHOxXDRlA     ^207 

chromosomes,  and  generation  after  generation  of 
such  females  appear  during  the  summer;  l)ut  in 
the  autumn,  females,  whose  eggs  must  be  fertilized 
before  they  will  develop,  and  males  are  produced. 
The  chromosomes  of  these  eggs  are  distributed  during 
maturation  as  shown  in  the  diagram  (Fig.  7^2). 
The  eggs  that  develop  into  the  females  possess  the 
usual  number  of  chromosomes,  but  those  that  give 
rise  to  males  cast  out  in  the  polar  body  one  chromo- 
some that  fails  to  divide,  and  hence  are  provided 
with  one  chromosome  less  than  the  others.  During 
the  maturation  of  the  germ  cells  of  these  males  two 
sorts  of  spermatozoa  are  formed,  one  with  three 
chromosomes,  the  other  with  only  two ;  the  latter 
degenerate.  Therefore,  since  only  one  sort  of 
spermatozoa  is  functional,  the  fertilized  winter 
eggs  are  all  alike  and  all  give  rise  to  females  (stem- 
mothers)  the  following  spring. 

The  chromosome  distribution  in  certain  nema- 
todes resembles  somewhat  that  of  the  phylloxerans. 
Here,  however,  we  have  to  deal  with  organisms  that 
are  peculiar  in  several  respects.  Maupas  (1900) 
has  shown  that  in  the  genus  RhahcUiis  the  number 
of  males  per  1000  females  ranges  from  45.0  to  0.15 
according  to  the  species ;  and  that  these  few  males 
do  not  copulate  with  the  females  and  hence  are  func- 
tionless.  Furthermore,  the  females  are  not  true 
females,  but  hermaphrodites.  Kruger  (1912)  dis- 
covered that  in  Rhahditis  aherrcuis  the  nuclei  of  the 
spermatozoa  did  not  fuse  with  that  of  the  v^^:^^,  except 
in  one  instance,  but  disappeared  in  the  cytojilasm  ; 


268 


GERM-CELL  CYCLE   IN  ANIMALS 


hence  the  spermatozoa  simply  initiate  development. 
The  chromosome  cycle  of  Rhabditis  nigrovenosa  has 
been  studied  by  Boveri  (1911)  and  Schleip  (1911). 

rHYLLOXERA    CARYMCAVLJS 

T'cCiA.  T'^:cac^  A    A    Q\ 


*y«rrut/   t^  ft2ft^eii   -^sn. 


o 


o 


i 


C^D 


c:rP 


o 


1 


<Seouu(/ J^/z/ey       yO    Ik^ 


o 


10 


Ti'^a/f  Sfii/ruiU. 


\ 


o  )  j-^^ 

Sf>t/mtalcaftc 


Fig.  72.  —  Chromosome  cycle  in  Phylloxera  carycBcaulis.     (From  Mor- 
gan's Heredity  and  Sex,  published  by  the  Columbia  University  Press.) 


CHROMOSOMES  AND   MITOCHONDRIA     269 

This  nematode  is  a  parasite  in  the  king  of  the  frog 
for  part  of  its  Hfe  cycle;  during  tliis  period  it  re- 
sembles the  female,  but  is  really  hermaphroditic. 
These  hermaphrodites  give  rise  to  free-living  indi- 
viduals which  are  true  males  and  females;  the 
eggs  of  the  latter  when  fertilized  develop  into  para- 
sitic hermaphrodites.  The  oogonia  and  sperma- 
togonia of  the  hermaphroditic  parasites  possess 
twelve  chromosomes  (Fig.  73,  ^1).  The  nucleus  of 
the  mature  egg  is  provided  with  six  (B) .  Two  sorts 
of  spermatozoa  are  formed,  one-half  with  six  chromo- 
somes, the  other  half  with  five ;  the  latter  result  from 
the  casting  out  of  one  chromosome  (E)  in  a  manner 
similar  to  that  described  above  in  Phylloxera.  The 
eggs  fertilized  with  the  spermatozoa  containing 
six  chromosomes  (F)  produce  free-living,  true  fe- 
males, whereas  those  fertilized  by  the  spermatozoa 
with  five  (G)  develop  into  free-living,  true  males. 
The  hermaphroditic  condition  is  regained  as  follows : 
The  free-living  females  give  rise  to  eggs  all  with 
six  chromosomes ;  the  males,  whose  spermatogonia 
contain  eleven  chromosomes,  produce  spermatozoa 
with  six  or  five  chromosomes;  those  with  the  latter 
number,  however,  are  not  functional,  hence  all 
fertilized  eggs  must  be  provided  with  twelve  chromo- 
somes and  develop  into  the  hermaphroditic  parasites. 
The  chromosome  cycle  in  pteropod  mollusks  as 
worked  out  by  Zarnik  (1911)  seems  even  more  re- 
markable than  that  described  for  nematodes.  The 
hermaphroditic  species,  C resets  acicula,  possesses 
twenty  chromosomes,  sixteen  large  ordinary  chromo- 


270 


GERM-CELL   CYCLE  IN  ANIMALS 


Fig.  73.  —  Rhabditis  nigrovenosa.  Stages  in  maturation,  fertilization, 
and  cleavage.  A.  Oogonium  with  twelve  chromosomes.  B.  Sec- 
ond maturation  division.  Pronucleus  and  second  polar  body  each 
with  six  chromosomes.  C.  Primary  spermatocyte.  D.  Division  of 
primary  spermatocyte.  E.  Second  spermatocyte  division ;  one 
chromosome  delayed.  F.  Two  spermatozoa  each  with  six  chromo- 
somes. G.  Cleavage  spindle  of  egg  showing  two  groups  of  chromo- 
somes ;  one  with  six  contributed  by  the  egg,  the  other  with  five 
contributed  by  the  sperm.     {After  Schleip,  1911.) 


CHROMOSOMES  AND   MITOCIIOXDRIA     ^71 


somes  (shown  in  black  in  Fig.  74),  two  lar^e  sex- 
chromosomes   (dotted),   and  two  small   sex-clironio- 


Spermato- 
gonium 


Spermato- 
cyte 1.  Ordn. 


Spermato- 
cyte 2.  Ordn 


Spermien 


Oogonium. 


/^^p^\\  Oocyte  1.  Ordn. 


C      /•  J  Oocyte  2.  Ordn. 


Reifes  Ei. 


Fig.  74.  — Diagrams  showing  the  chromosome  cycle  in  the  pteropod 
mollusk,  Creseis  acicula.  In  order  to  sinipHfy  thi-  (Ha^rams  each 
black  chromosome  is  made  to  represent  ei^ht  ordinary  chromosomes. 
{After  Zarnik,  1911.) 

somes  (dotted).     The  spermalo^'oiiia  outer  the  mat- 
uration period  in  this   condition.     The    nuniljcr   of 


272        GERM-CELL   CYCLE  IN  ANIMALS 

chromosomes  is  reduced  in  the  first  division,  resulting 
in  two  secondary  spermatocytes  each  with  eight 
large  ordinary  chromosomes,  and  one  large  and  one 
small  sex-chromosome.  During  the  second  division 
the  small  sex-chromosome  does  not  divide,  but  passes 
intact  into  one  spermatid ;  thus  two  sorts  of  sperma- 
tozoa are  formed,  one  with  eight  large  ordinary  and 
one  sex  chromosome  and  the  others  with  eight 
large  ordinary  chromosomes  and  two  large  sex- 
chromosomes.  The  spermatozoa  with  only  one 
sex  chromosome  is  not  functional.  The  oogonia 
differ  from  the  spermatogonia  and  somatic  cells  in 
the  possession  of  sixteen  large  ordinary  chromosomes 
and  four  small  sex-chromosomes ;  two  of  the  latter 
arise  by  the  diminution  of  the  chromatin  in  two  of 
the  large  sex-chromosomes.  The  maturation  divi- 
sions are  of  the  usual  sort,  and  all  of  the  eggs  are 
alike,  containing  eight  large  ordinary  chromosomes 
and  two  small  sex-chromosomes.  Fertilization,  as 
indicated  in  Fig.  74,  always  results  in  a  zygote  with 
sixteen  large  ordinary  chromosomes,  two  large  sex- 
chromosomes,  and  two  small  sex-chromosomes,  which 
develop  into  a  hermaphroditic  individual. 

Although  we  know  very  little  about  the  chromo- 
somes of  man,  the  data  available  seem  to  indicate 
that  here  also  there  are  chromatin  bodies  concerned 
with  sex-determination.  The  following  table  indi- 
cates the  state  of  our  knowledge  at  the  present  time. 

Guyer  (1910)  was  the  first  to  announce  the  dis- 
covery of  accessory  chromosomes  in  man.  He  found 
twenty-two    chromosomes    in    the    spermatogonia, 


CHROMOSOMES   AND   MITOCIIOXDRIA     273 


Table  Showing  the  Number  of  Chromosomes  in  Man 

ACCORDING   TO    VaRIOUS   INVESTIGATORS 


Diploid 
Number 

Haploid  Number 

Investigator 

Date 

Bardelehen 

1892 

24 

Flemming 

1897 

18  (15  or  19)  1 

Wilcox 

1900 

12 

Duesberg 

190(1 

32 

Farmer,  Moore,  and  AValker 

1900 

16 

Moore  and  Arnold 

1900 

12  or  10 

Giiver 

1910 

12  or  10 

Montgomery 

1912 

24(?) 

Gutherz 

1912 

47 

23  or  24 

Winiwarter 

19  b2 

34  (33,  38) 

Wieman 

1913 

which  became  ten  bivalent  and  two  accessories  in 
the  primary  spermatocytes.  The  latter  pass  un- 
divided to  one  pole  (Fig.  75,  A),  and  hence  two  classes 
of  spermatozoa  result,  one  with  ten  ordinary  chromo- 
somes, and  the  other  with  ten  ordinary  and  two 
accessory  chromosomes.  Winiwarter  (1912),  on  the 
other  hand  (Fig.  75,  D-E),  reports  forty-seven 
chromosomes  in  the  spermatogonia  and  two  classes 
of  spermatozoa,  one  with  twenty-three  and  the 
other  with  twentv-four.  The  number  in  the  female, 
according  to  Winiwarter,  is  probably  forty-eight, 
and  hence  all  mature  eggs  are  alike  so  far  as  chromo- 
some number  is  concerned,  each  being  provided 
with  twentv-four.  If  these  data  are  coniirmed,  it  is 
evident  that  sex  in  man  is  determined  at  the  time 
of  fertilization  and  cannot  be  influenced  by  changing 
the  environment. 

1  Wilcox  doesn't  state  whether  this  is  the  reduced  or  diploid  number. 
T 


274        GERM-CELL  CYCLE   IN  ANIMALS 

The  above  illustrations  indicate  that  there  is  some 
internal  mechanism  which  controls  sex,  and  that 
certain  chromosomes  are,  in  at  least  many  cases, 

yi',"':  »■•,■■.«";-"•■**•. 


•■'-—.> 


\-sxy: 


Fig.  75.  —  Chromosomes  in  man.  A.  First  spermatocyte  division  show- 
ing two  accessories  passing  early  to  one  pole.  B.  Two  contiguous 
spermatids,  one  without  and  the  other  with  two  accessories.  C.  Two 
secondary  spermatocytes  ;  the  one  above  with  an  accessory.  D.  Sec- 
ond spermatocyte  with  twenty-four  dyads.  E.  Second  spermatocyte 
with  twenty-three  dyads.  \A-B,  from  Guyer,  1910;  C-E,  from 
Winiwarter,  1912.) 


factors  in  sex-determination.  Several  hypotheses 
have  been  suggested  as  to  the  relation  of  these 
chromosomes  to  sex,  such  as  that  sex  is  determined 
by  the  quantity  of  chromatin  present  in  the  zygote. 


CHROMOSOMES   AND   MITOC  IIOXDRI  V     ^275 

No  view,  however,  has  won  general  acceptance,  but 
it  seems  probable  that  there  are  fundamental  inter- 
relations between  the  different  parts  of  the  cell  which 
regulate  the  behavior  of  the  chromosomes.  We 
must,  therefore,  look  further  for  an  explanation  of 
sex-determination.  It  has  been  suggested  that 
differences  in  metabolism  may  be  responsible  for  the 
fundamental  differences  between  the  sexes.  Ac- 
cording to  this  view  changes  in  metabolism  may 
control  the  behavior  of  the  sex-chromosomes,  or  the 
presence  of  the  sex-chromosomes  in  every  cell  in 
the  body  may  influence  the  metabolism  "in  such  a 
way  that  the  organism  is  caused  to  become  of  one 
sex  rather  than  of  the  other,  in  consequence  of  its 
type  of  metabolism  "   (Doncaster,  1914,  p.  515). 

The  Mitochondria  of  Germ  Cells 

The  study  of  the  relative  importance  of  the  nucleus 
and  the  cytoplasm  in  heredity  has  been  given  a  new 
impetus  within  recent  years  by  the  more  accurate 
examination  and  description  of  certain  cytoplasmic 
inclusions  of  both  germ  cells  and  somatic  cells  known 
as  mitochondria,  chondriosomes,  plastosomes,  chro- 
midia,  etc.  Some  of  the  best  recent  evidence  that 
part  of  the  germ-plasm  may  be  located  in  the  cyto- 
plasm is  afforded  by  the  work  of  Bcnda,  Meves, 
Regaud,  Duesberg,  and  others  on  the  history  of 
these  mitochondrial  bodies  during  maturation,  fer- 
tilization, early  cleavage,  and  cellular  differentiation. 
As  long  as  forty  years  ago  the  cytoplasm  of  the 
germ  cells  was  known  to  contain  bodies  other  than 


276        GERM-CELL  CYCLE   IN  ANIMALS 

the  nucleus ;    these  bodies  have  been  given  various 
names  such  as  spherules   (Kunstler,  1882),  cytomi- 
crosomes  (La  Valette    St.   George,    1886),  bioblasts 
(Altmann,  1890),  and  ergastoplasm    (Bouin,   1898). 
In  1897  and  1898  Benda  noticed  the  constant  pres- 
ence of  certain  granules  in  the  male  germ  cells  of  a 
number  of  vertebrates  and  was  able  to  trace  their 
history  from  the  spermatogonia  until  they  formed  the 
spiral  filament  in  the  tail  of  the  spermatozoa.     These 
observations     were    extended    the    following    year 
(1899)  so  as  to  include  all  stages  in  the  development 
of  the  eggs  and  spermatozoa  of  many  vertebrates 
and  invertebrates  and  also  various  tissue  cells  such 
as   striated   muscle-fibers,   leucocytes,   marrow-cells, 
etc.     This    work    attracted    wide    attention    chiefly 
for  two  reasons:     (1)  the  history  of  the  granules 
was  carefully   worked  out  and  the  various  stages 
accurately  described,   and   (2)  special,   rather  com- 
plicated, staining  methods  were  devised  which  were 
supposed   to   color   the   mitochondria   so   that   they 
could  be  distinguished  from  all  other  cell  inclusions. 
From  1899  until  the  present  time  an  ever  increasing 
number  of  investigators  have  attacked  the  problems 
presented  by  the  mitochondria,  or  referred  to  these 
structures  incidentally  when  working  upon  other  his- 
tological or  cytological  problems.     The  study  of  mito- 
chondria   received    its    greatest    impetus,    however, 
in  1908,  when  Meves  published  a  paper  on  these 
structures  in  the  chick  embryo  entitled  ''Die  Chon- 
driosomen  als  Trager  erblicher  Anlagen."     In   this 
paper  the  chick  embryo  is  described  from  the  fifteen- 


CHROMOSOMES  AND   MITOCHONDRIA     2 


■:  /  / 


hour  stage  up  to  the  three-days-nine-liour  si  age. 
The  cells  of  the  earHest  stage  studied  contained  mito- 
chondria (Fig.  76)  which  were  differently  arranged 
in  the  germinal  layers :  the  ectoderm  and  entoderm 
cells  contained,  for  the  most  part,  rods  and  threads, 
the  granules  being  scarce,  and  the  mesoderm  cells 
were  characterized  by  numerous  granules  and  few 
rods  and  threads.  At  the  three-day  stage  the  mito- 
chondria of  the  neuroblasts  became  difficult  to 
stain  by  the  usual  method,  but  did  stain  like  neuro- 
fibrils. These  and  other  observations  led  Meves 
to  the  conclusion  that  the  mitochondria  are  of  con- 
siderable importance  in  cellular  differentiation  and 
are  in  fact  the  bearers  of  hereditary  Anlagen. 

Since  this  paper  of  Meves  appeared,  the  zoological 
periodicals  have  been  flooded  with  the  results  of  in- 
vestigations of  the  mitochondria  in  almost  every 
sort  of  germ  and  somatic  cell,  both  normal  and 
abnormal,  and  in  Protozoa  and  Metazoa,  In- 
vertebrates and  Vertebrates.  No  report  on 
spermatogenesis,  oogenesis,  or  early  embryonic  de- 
velopment is  complete  without  reference  to  the  mito- 
chondria. In  plants,  also,  cellular  bodies  have  been 
described  of  a  mitochondrial  nature  (Meves,  1904  ; 
Duesberg   and   Hoven,  1910;    Guilliermond,  1911). 

A  large  number  of  new  terms  have  been  coined 
for  the  purpose  of  describing  these  cytoj)lasniic  in- 
clusions. Some  of  them  are  as  follows:  (1)  mito- 
chondria, applied  by  Benda  (1897,  189cS)  to  certain 
granules  with  definite  staining  reactions :  ('2)  chon- 
driosomes,  proposed  by  jVIeves  (1908)  for  both  single 


278        GERxM-CELL   CYCLE   IN  AxNIMALS 


^'"'■ll--}-^}^'''^.^^^^^  in   the  embryonic  cells  of  the  chick      A    In 
cells  of  the  primitive  strenk-       R     Jt^a;.-^-  "^c  luilk.     jt.  m 

C    In  connective  tissue  cet,     D.  Int"a'  "LrSf 'T'Z^  'f^' 
oerg,  wi^,   a,  B,  C,  E,  after  Meves;  D,  F,  after  Duesherg.) 


CHROMOSOMES   AND   MITOCHONDRIA    279 

granules  and  chains  of  granules ;  the  latter  were  also 
called  chondriokonts ;  (3)  plastosomes  (plastochon- 
dria,  plastokonta),  employed  by  Meves  (1910)  be- 
cause of  their  supposed  role  in  histogenesis ;  (4) 
eclectosomes,  selected  by  Regaud  (1909)  as  a  general 
physiological  expression  for  chondriosonies  ;  (5)  chon- 
driotaxis,  used  by  Giglios-Tos  and  Granata  (1908) 
to  describe  the  parallel  arrangement  of  chondrio- 
konts ;  (6)  chondriodierese,  proposed  by  the  same 
authors  for  the  division  of  the  chondriokonts  during 
cell  division  ;  (7)  karyochondria,  coined  by  AVildman 
(1913)  for  cytoplasmic  inclusions  derived  from  the 
basichromatin  of  the  nucleus ;  (8)  chromidia,  a  term 
considered  by  Goldschmidt  (1904)  and  others  to  in- 
clude the  mitochondria. 

We  are  here  especially  interested  in  the  mitochon- 
dria of  the  germ  cells,  their  origin,  fate,  and  signif- 
icance, but  our  ideas  regarding  the  importance  of 
these  bodies  in  heredity  depend  somewhat  upon  their 
behavior  in  somatic  cells.  As  already  stated, 
Benda  (1903)  observed  mitochondria  in  both  germ 
cells  and  somatic  cells.  Since  then  they  have  been 
recorded  in  Protozoa,  in  almost  every  sort  of  somatic 
cell  in  Metazoa,  and  in  many  plant  cells  (Fig.  77). 
Excellent  reviews  have  been  published  by  Benda 
(1903),  Faure-Fremiet  (1910),  Prenant  (1910),  and 
Duesberg  (1912).  These  reviews  have  led  to  the 
conclusion  already  expressed  by  Regaud  (1909, 
p.  920)  that  "it  is  probable  that  they  (mitochondria) 
exist  in  all  cells,  at  least  at  certain  stages  in  their 
activities."     Among  the  somatic  differentiations   to 


280 


GERM-CELL  CYCLE  IN  ANIMALS 


which  mitochondria  are  supposed  to  give  rise  are 
neurofibrils  and  myofibrils.  Meves  (1907,  1908) 
considered  it  probable  that  neurofibrids  were  trans- 
formed chondriosomes,  and  Hoven  (1910)  seemed  to 
have  proved  it,  but  Marcora  (1911)  and  Cowdry 
(1914)  find  that  the  neurofibrils  arise  independently, 


^ 


^yJi. 


A  B  C 

Fig.  77.  —  Mitochondria  in  the  cells  of  a  plant,  Pisum  sativum. 
A.  Young  germ  cell.  B.  Young  germ  cell  dividing.  C.  Old  cell 
containing  vacuoles.     {From  Duesberg  and  Hoven,  1910.) 

although  mitochondria  are  present  in  the  nerve 
cells.  Duesberg  (1910)  is  quite  positive  that  the 
myofibrils  of  striated  muscle  fibers  are  produced 
by  the  metamorphosis  of  chondriosomes  from  em- 
bryonic muscle  cells,  and  has  recently  (Duesberg, 
1913)  strengthened  his  position  by  the  discovery 
that  the  myoplasm  described  by  Conklin  (1905) 
in  the  egg  of  the  Ascidian,  Cynthia,  is  well  supplied 
with  chondriosomes. 

Mitochondrial  structures  have  been  studied  in 
both  living  and  preserved  cells.  Faure-Fremiet 
(1910)  describes  them  in  living  cells  (Fig.  78,  D)  as 


CHROMOSOMES   AND   MITOCHOXDRIA     281 

small,  transparent,  slightly  refringent  granuk's  of  a 
pale  gray  tint,  either  homogeneous  or  else  vesicular 
with  fluid  contents  and  a  thin,  denser,  rci'ringcuL 
periphery.  Rod-like  mitochondria  were  likewise 
observed  by  Montgomery  (1911)  in  the  living  male 
germ  cells  of  Euschistus  (Fig.  78,  A-B)  which  had 
been  teased  out  in  Ringer's  solution ;    and  this  in- 


A  O 

Fig.  78.  —  Division  of  mitochondria.  A-B.  Mitochondrial  rods  divid- 
ing during  first  maturation  division  in  Euschistus.  C.  Stages  in 
division  of  mitochondrial  l)ody  in  Hudromctra.  D.  Sinniltnnt'oua 
division  of  micronucleus  and  mitochondria  in  Carchcsium  (in  vivo). 
(A-B,  from  Montgomery,  1911;  C,  from  Wilke,  1913;  D,  from 
Faure-Fremiet,  1910.) 

vestigator  concluded  that  in  preserved  material  "we 
have  been  working  with  images  that  are  very  close 
to  the  living.  ..."  More  recently  Lewis  [{\u\ 
Lewis  (1914)  have  made  careful  .studies  of  niiloclion- 
dria  in  living  cells  from  chick  embryos.  Granules 
were  here  seen  *'to  fu.se  together  into  rods  or  chaius, 
and  these  to  elongate  into  threads,  wliich  iu  turn 
anastomose  with  each  other  and  may  nnilc  into  a 
complicated  network,  which  iu  turn  may  again 
break  down  into  threads,  rods,  loops,  and  rings." 
Even  more  remarkable  are  the  movements  within  the 


282        GERM-CELL   CYCLE  IN  ANIMALS 

cell  described  by  the  same  investigators.  "The 
mitochondria  are  almost  never  at  rest,  but  are  con- 
tinually changing  their  position  and  also  their  shape. 
The  changes  in  shape  are  truly  remarkable,  not  only 
in  the  great  variety  of  forms,  but  also  in  the  rapidity 
with  which  they  change  from  one  form  to  another.  A 
single  mitochondrium  may  bend  back  and  forth 
with  a  somewhat  undulatory  movement,  or  thicken 
at  one  end  and  thin  out  at  the  other  with  an  appear- 
ance almost  like  that  of  pulsation,  repeating  this 
process  many  times.  Again,  a  single  mitochondrium 
sometimes  twists  and  turns  rapidly  as  though 
attached  at  one  end,  like  the  lashing  of  a  flagellum, 
then  suddenly  moves  off  to  another  position  in  the 
cytoplasm  as  though  some  tension  had  been  re- 
leased." Mitochondria  may  also  be  stained  intra 
vitam,  especially  with  dahlia  violet  and  Janus  green. 
Most  of  the  fixing  solutions  ordinarily  used  for  cy to- 
logical  purposes  destroy  the  mitochondria.  The 
methods  which  seem  to  give  the  best  results  have 
osmic  acid  or  formalin  as  a  basis,  such  as  those  de- 
vised by  Altmann  (see  Lee,  1905,  p.  43),  Benda 
(Lee,  1905,  p.  223),  Meves  (1908),  and  Regaud 
(1908,  p.  661).  Benda  (1903)  claimed  that  all 
cellular  structures  which  stained  violet  by  his  method 
were  of  a  mitochondrial  nature  ;  but  this  has  not  been 
found  to  hold  true.  Undoubtedly  the  many  bodies 
which  have  been  discovered  in  cells  are  of  several 
sorts,  and  only  by  a  thorough  study  of  their  staining 
qualities,  morphological  aspects,  and  biological  roles 
can    they    be    identified.     Benda's    crystal    violet 


CHROMOSOMES   AND   MITOC  IIOXDRIA    283 

stain  seems  to  be  more  selective  than  any  other  for 
mitochondria  and  is  of  great  value  for  this  reason. 

Mitochondria  most  often  appear  as  spherical 
or  elongated  granules  about  0.001  mm.  diameter. 
These  granules  may  become  arranged  in  a  series, 
thus  forming  a  chain,  and  the  granuK's  in  a  chain 
may  fuse  into  a  homogeneous  rod.  Different  forms 
are  present  in  different  kinds  of  cells  or  even  in  the 
same  cell  at  various  stages  in  its  evolution  or  func- 
tional activity.  Some  investigators  (Prenanl.  1!)10) 
maintain  that  the  homogeneous  rod  is  the  i)rimitive 
condition  and  that  the  granules  are  formed  by  the 
disintegration  of  such  rods;  to  others  just  the 
reverse  seems  to  be  true  (Rubaschkin,  1910;  Dues- 
berg,  1912). 

The  chemical  constitution  of  the  mitoelionchia 
has  been  studied  by  a  number  of  investigators. 
Regaud  (1908)  has  shown  that  the  miloehondria  of 
the  seminal  epithelium  are  not  histocluMnically 
identical.  He  distinguishes  three  sorts  of  granuk's  : 
(1)  those  which  resist  the  action  of  acetic  acid  and 
are  stainable  without  being  previously  innncrsed  in 
a  solution  of  potassium  bichroniale,  (2)  granuk's 
which  resist  acetic  acid  but  require  int(Mis(^  ctiromisa- 
tion,  and  (3)  granules  which  (k)  not  resist  acetic- 
acid  and  demand  ckromisation.  Faure-Frenn'el , 
Mayer,  and  Scliiiffer  (1<)09)  have  studied  the  mito- 
chondria by  microchemical  and  conipaiali\c  niclliods 
and  reached  the  conclusion  that    llicy   are  lecillial- 

bumins. 

Mitochondria   have   been   noted    in    all    stages   of 


284 


GERM-CELL   CYCLE   IN   ANIMALS 


the  male  germ-cell  cycle,  especially  in  mammals, 
mollusks,  and  insects,  and  appear  to  be  continuous 
from  one  generation  of  cells  to  the  next.     During 


Fig.  79.  —  Behavior  of  the  mitochondria  during  the  fertilization  and 
early  cleavage  of  the  egg  of  Ascaris.  A.  Egg  into  which  a  sperma- 
tozoon has  penetrated.  B,  C.  The  mixing  of  the  mitochondria  of 
the  egg  and  spermatozoon.  D.  Division  stage  of  the  first  two  blas- 
tomeres.     (After  Meves,  1911  and  1914.) 

mitosis  the  plastosomes  lie  outside  of  the  spindle 
(Fig.  79,  D)  ;  they  may  divide  autonomously  as 
claimed  by  Faure-Fremiet  (1910)  in  Protozoa  (Fig. 


CHROMOSOMES   AND    MITOCHONDRIA     ^285 

78,  D)  and  Wilke  (10V2)  in  tlie  spermatocytes 
of  Ilydrometra  or  en  masse,  as  in  the  spermatogenesis 
of  Euschistus  (Fig.  78,  A-B),  thus  undergoing  a  sort 
of  paramitosis  (Montgomery,  1911)  and  Xofouecfa 
(Browne,  1913).  In  the  former  cases  each  (hiughter 
cell  is  supposed  to  receive  one-half  of  each  gramde; 
in  the  latter  the  distribution  is  largely  by  chance, 
but  apparently  equal  (Cowdry,  1914).  Accor( Hug 
to  certain  observers  the  centrosomes  exert  an  in- 
fluence upon  the  mitochondria  as  indicated  by  the 
aggregation  of  these  bodies  around  the  asters  (Faure- 
Fremiet,  1910;  Meves,  19U)  ;  but  others  have  been 
unable  to  find  any  confirmatory  evidence  in  their 
material  (Montgomery,  1911).  Duesberg  (1908) 
has  pointed  out  that  since  there  is  no  rest  period 
between  the  two  maturation  divisions  there  nuist  be 
a  quantitative  reduction  of  plastosomes  in  the  sj)er- 
matids;  a  quartering  of  the  mitochondria  couKl 
not,  however,  be  observed  by  :Montgomery  (191 '■2) 
in  Peripatus.  Montgomery  (1911)  has  suggested 
that  the  relative  amount  of  the  mitochondrial  sub- 
stance received  ''might  determine  the  sex-i)rcp()n- 
derance  character  of  the  sperm,  a  matter  unfor- 
tunately very  difficult  to  test." 

Faure-Fremiet  recognizes  four  ty]H\s  of  mitochon- 
drial distribution  in  the  germ  cells:  (1)  filaments  or 
masses  that  do  not  undergo  i)r()found  mori)h()logical 
changes  (Fig.  80)  ;  (2)  one  or  more  masses  which 
transform  into  a  definite  morphological  element, 
the  Nebenkern;  (3)  masses  which  only  ])artially 
change  into  a  Nebenkern  or  yolk  nucleus  ;   (4)   bodies 


286 


GERM-CELL   CYCLE   IN  ANIMALS 


which   transform   entirely    or   in    part    into    deuto- 
plasmic  granules  of  a  fatty  nature. 

The  origin  of  the  mitochondria  in  male  cells  can- 
not be  stated  definitely,  since  certain  investigators 
(Goldschmidt,  Buchner,  Wassilieff,  etc.)  claim  that 
they  arise  from  the  nucleus ;  others  (e.g.,M.eves,  Wilke, 
Duesberg)  consider  them  to  be  integral  parts  of  the 

cytoplasm  ;  and  a  third 
group  (Montgomery, 
Browne,  Wildman) 
looks  upon  some  of 
them  as  the  results 
of  chemical  interaction 
between  the  nucleus 
and  the  cytoplasm. 

Less  is  known  con- 
cerning the  mitochon- 
dria during  oogenesis 
than  during  sperma- 
togenesis, but  certain  bodies  have  been  described  in 
the  ova  of  a  number  of  animals  which  exhibit  all  of 
the  characteristics  of  the  mitochondria  of  male  cells. 
As  in  the  latter,  they  have  been  considered  chromidial 
by  some  and  of  cytoplasmic  origin  by  others. 

The  importance  of  the  mitochondria  depends 
largely  upon  their  functions.  Those  of  the  egg  have 
been  observed  by  Russo  (1907),  Loyez  (1909), 
Faure-Fremiet  (1910),  Van  Durme  (1914),  Hegner 
(1914a),  and  others  to  transform  directly  into  yolk 
globules.  According  to  Van  der  Stricht  (1904), 
Lams  (1907),  etc.,  they  produce  yolk  elements  in- 


FiG.  80.  —  Four  stages  in  the  formation 
of  the  spermatozoon  of  Enteroxenos 
showing  the  distribution  of  the  mito- 
chondria (M).     {After  Bonnevie.) 


CHROMOSOMES   Ax\l)    MlTO(  IIONDUIA     ^287 

directly;  and  It  is  the  opinion  of  Movos,  Dueshcr^S 
and  their  followers  that  they  play  an  iniporlaiit  mle 
in  fertilization.  Likewise  in  the  spermatozoa  ideas 
differ  regarding  their  functions.  I^cnda  (IS!)!)) 
believed  them  to  be  motor  organs;  Kollzofl'  (l!)(l(i), 
from  a  study  of  the  spermatozoa  of  Decapods, 
maintains  that  they  represent  elements  whidi  form 
a  sort  of  cellular  skeleton;  Rcgaud  (1J)()!))  claims 
that  they  are  the  particular  crlhihir  organs  which 
exercise  a  *'fonction  eclectique,"  extracting  and 
fixing  substances  in  the  cell,  and  should  Iherefore  he 
called  "  eclectosomes " ;  and  Meves  (1907,  1908) 
holds  that  they  are  cytoplasmic  constituents  cor- 
responding to  the  chromosomes  of  the  nuch'us. 
Meves  (1907,  1908)  came  to  the  conclusion  that  tliere 
must  be  hereditarj^  substances  in  the  cyl()j)la>ni, 
and  by  the  method  of  elimination  decide(j  in  faxor 
of  the  mitochondria.  In  his  stu(hes  on  fertihzation 
and  cleavage  in  Ascaris  (Meves,  1911.  1!)M-)  he  has 
shown  that  granules  from  the  spermatozoon  (Fig. 
79)  fuse  with  similar  granules  in  the  v^^:,^^,  as  described 
previously  by  L.  and  R.  Zoja  (1891),  and  thai  (lie.se 
granules  are  plastosomes.  The  distiibntion  of  the 
fused  granules  is  followed  until  the  am|)hiaster  is 
formed  in  the  two-cell  stage;  here  the  ])lastosomes 
are  mainly  grouped  about  the  centrosomes,  although 
a  few  are  scattered  about  in  the  cytoplasm  (Fig.  79, 
D). 

Although  there  are  many  who  believe  Meves  and 
his  followers  to  be  correct  in  their  contention  that 
the  plastosomes  are  the  bearers  of  hereditary  charac- 


288        GERM-CELL   CYCLE   IN  ANIMALS 

teristics  in  the  cytoplasm,  just  as  the  chromosomes  are 
the  bearers  of  hereditary  characteristics  in  the 
nucleus,  still  there  are  many  objections  to  this  view, 
such  as  the  fact  that  part  or  all  of  the  plastosomes 
may  be  cast  out  of  the  spermatid  {e.g.,  in  the  opos- 
sum, Jordan,  1911 ;  and  in  Peripatus,  Montgomery, 
1912).  It  is  obvious  from  the  foregoing  account 
that  there  are  a  number  of  opposing  views  regarding 
the  origin,  nature,  and  role  of  the  various  cytoplasmic 
inclusions  which  have  been  considered  mitochondria. 
Are  they  constant,  necessary  constituents  of  the 
living  protoplasm,  or  are  they  inactive  lifeless  bodies 
which  may  be  included  under  the  term  metaplasm  ? 
If  they  constitute  a  part  of  the  living  protoplasm, 
do  they  form  the  skeleton  of  the  cell,  do  they  take 
part  in  the  metabolic  activities  of  the  cytoplasm 
or  nucleus,  or  do  they  play  a  role  in  the  process  of 
differentiation,  and  should  they  be  considered  as 
the  hereditary  substance  of  the  cytoplasm  ?  If 
they  are  simply  metabolic  products,  are  they  excretory 
in  nature,  or  reserve  materials  set  aside  for  the  later 
use  of  the  cell  ?  And  finally,  do  they  arise  from  the 
nucleus,  are  they  strictly  cytoplasmic,  or  do  they 
originate  through  the  interaction  of  nucleus  and 
cytoplasm  ?  It  is  impossible  in  a  short  space  to 
give  an  adequate  account  of  the  arguments  pro  and 
con,  and  so  we  must  refer  the  reader  to  the  compre- 
hensive reviews  mentioned  above.  The  conclusion, 
however,  is  perfectly  safe  that  we  shall  have  to  await 
the  results  of  further  investigations  before  we  can 
come  to  a  definite  decision.     In  the  meantime  we 


CHROMOSOMES  AND    M1T()(  IIONDKIA     289 

should  thank  the  mitochondria  for  focusing  llie 
attention  of  cytologists  upon  tlie  cytoplasnn'c  cl(»- 
ments,  since  the  belief  is  becoming  more  and  more 
general  that  hereditary  phenomena  are  the  result  of 
interactions  between  nucleus  and  cytophism  and  that 
the  latter  may  play  a  more  important  role  than  is 
usually  supposed. 


CHAPTER  X 

THE    GERM-PLASM   THEORY 

In  discussing  the  germ-plasm  theory  it  is  necessary 
to  distinguish  between  this  hypothesis  and  that  of 
the  morphological  continuity  of  the  germ  cells.  The 
facts  and  theories  involved  have  grown  up  to- 
gether. Owen  (1849)  was  perhaps  the  first  to 
point  out  the  differences  between  germ  cells  and  body 
cells.  "Not  all  of  the  progeny  of  the  primary  impreg- 
nated germ  cell, "  he  writes,  "are  required  for  the  for- 
mation of  the  body  in  all  animals ;  certain  of  the  de- 
rivative germ  cells  may  remain  unchanged  and  become 
included  in  that  body  which  has  been  composed  of 
their  metamorphosed  and  diversely  combined  and 
confluent  brethren;  so  included,  any  derivative 
germ  cell  or  the  nucleus  of  such  may  commence  and 
repeat  the  same  processes  of  growth  by  imbibition, 
and  of  propagation  by  spontaneous  fission,  as  those 
to  which  itself  owed  its  origin.  ..."  Galton  (1872) 
was  among  the  earliest  to  recognize  the  necessity 
for  two  sorts  of  materials  in  the  individual  metazoon, 
"one  of  which  is  latent  and  only  known  to  us  by  its 
effects  on  his  posterity,  while  the  other  is  potent, 
and  constitutes  the  person  manifest  to  our  senses." 
He  at  that  time  believed  in  the  inheritance  of  ac- 
quired characters  and  conceived  the  egg  as  a  struc- 

290 


THE    GERM  PLASM    TilKOin  291 

tureless  body  from  wliicli  holli  I  lie  body  .ind  the  ova 
of  the  individual  evolve;  and  considered  lliese  ova 
to  consist  of  contributions  partly  from  I  he  v<n^  and 
partly  from  the  body  which  develoix'd  from  the  e^'^'. 
Later  Jager  (1877)  stated  the  idea  of  gerim'nal  con- 
tinuity more  definitely.  He  maintained  that  |)art 
of  the  germ-plasm  (Keini  Protoplasma)  of  the 
animal  forms  the  individual,  and  the  rest  is  re- 
served until  sexual  maturity,  when  it  forms  the  re[)ro- 
ductive  material.  The  reservation  of  this  j)hyl()- 
genetic  substance  he  termed  the  "continuity  of 
the  germ-plasm"  ("Continuitiit  des  Keimproto- 
plasmas").  To  Weismann  (1885)  is  usually  givtMi 
the  credit  for  originating  the  germ-j)lasui  theory, 
but  while  w^e  are  undoubtedly  iiKh'bted  lo  him  for 
the  great  influence  the  hyj^othesis  of  germinal  con- 
tinuity has  had  upon  the  trend  of  biological  in\  (•>({- 
gations  within  the  past  thirty  years,  we  must  con- 
sider Jager  as  the  first  to  clearly  enunciate  I  he  i(N  a. 
Jager  (1878)  also  expressed  a  belief  in  the  mor- 
phological continuity  of  the  germ  cells  of  succeed- 
ing generations,  but  this  idea  was  first  definitely 
stated  by  Nussbaum  (1880),  whose  investigations 
of  the  germ  cells  in  the  trout  and  I'log  led  him  to 
conclude  that  the  cleavage  cells  t'orni  two  groups 
independent  of  each  other.  One  group  contains 
the  cells  which  multi})ly  and  dill'erent iate  and  Ihns 
build  up  the  body  of  the  individual,  but  do  not  |)i'o- 
duce  germ  cells;  the  other  group  takes  no  pari  in 
the  formation  of  the  body  and  undergoes  no  ditl'eren- 
tiations,  but  mult  ij)lics  by  simple  division.     The  germ 


292        GERM-CELL   CYCLE   IN  ANIMALS 

cells  are  thus  not  derived  from  the  individual  in 
which  they  lie,  but  have  a  common  origin  with  it. 
The  segregated  germ  cells  or  species  substance  is 
therefore  distinct  and  independent  of  the  individual ; 
this  accounts  for  the  constancy  of  the  species.  We 
may  distinguish  between  the  two  ideas  by  defining 
them  as  follows  : 

(1)  Germinal  continuity,  or  the  germ-plasm 
theory.  "In  each  ontogeny  a  part  of  the  specific 
germ-plasm  contained  in  the  parent  egg-cell  is  not 
used  up  in  the  construction  of  the  body  of  the  off- 
spring, but  is  reserved  unchanged  for  the  formation 
of  the  germ  cells  of  the  following  generation" 
(Weismann,  1891,  p.  170). 

(2)  Morphological  continuity  of  the  germ  cells. 
The  developing  egg  produces  by  division  two  sorts  of 
cells,  germ  cells  which  contain  the  germ-plasm  and 
somatic  cells  which  protect,  nourish,  and  transport 
the  germ  cells  until  they  leave  the  body  to  give 
rise  to  the  succeeding  generation. 

No  case  of  a  complete  morphological  continuity 
of  germ  cells  has  ever  been  described.  Such  an 
occurrence  would  necessitate  the  division  of  the  egg 
into  two  cells,  one  of  which  would  give  rise  to  all 
of  the  body  cells  and  nothing  else,  the  other  only  to 
germ  cells.  The  behavior  of  the  germ-plasm  in  such 
a  case  would  be  as  follows  (Weismann,  1904,  p.  410)  : 
"The  germ-plasm  of  the  ovum  first  doubles  itself 
by  growth,  as  the  nuclear  substance  does  at  every 
nuclear  division,  and  then  divides  into  two  similar 
halves,  one  of  which,  lying  in  the  primordial  somatic 


THE    GERM  PLASM    TIIKORV  293 

cell,  bec'oiiu's  al  oner  acllxc  and  breaks  up  into 
sinallor  and  smaller  ^r()U])s  oi"  (Ictcrnniiaiits  corre- 
sponding to  the  building  up  of  llic  Ixxly,  wliilc  tlir 
germ-plasm  in  the  other  ri'iuains  in  a  more  or  less 
'bonnd'  or  'set'  condition,  and  is  only  actJNc  to  the 
extent  of  gradually  stamping  as  germ  cells  the  cells 
which  arise  from  the  primordial  germ  cell." 

According  to  Weismann  this  actually  occurs  in 
Dipterous  insects,  but  there  is  no  evidence  in  the 
literature  to  warrant  this  statement.  It  is  conse- 
quently necessary  to  imagine  the  germ-plasm  as 
present  but  not  definitely  localized  in  a  germ  cell 
until  some  time  after  the  two-cell  stage  has  been 
reached.  Thus  in  hydroids  Weismanu  explains  the 
situation  as  follows:  "Here  the  j)rimordial  germ 
cell  is  separated  from  the  ovum  by  a  long  series  of 
cell-generations,  and  the  sole  possibility  of  explaining 
the  presence  of  germ-plasm  in  thi>  i)rimor(lial  germ 
cell  is  to  be  found  in  the  assumi)lion  that  in  the 
divisions  of  the  ovum  th(*  whole  oi'  the  gerni-pla>m 
originally  contained  in  it  was  not  broken  up  into 
determinant  groups,  but  that  a  i)art,  i)erhaps  the 
greater  part,  was  handed  on  in  a  latent  state  from 
cell  to  cell,  till  sooner  or  later  it  reached  a  cell  which 
it  stamped  as  the  primordial  germ  cell." 

Evidence  that  the  germ-])lasm  does  become  sooner 
or  later  localized  in  the  i)rimor(lial  germ  cell  has  accu- 
i  mulated  rai)idly  within  recent  years.       In  the  pa^lo- 

genetic  fly,  Miastor  (see  (1iai)ter  III),  the  fir>t  (vU 
to  be  cut  oflF  from  the  egg  is  the  i)rimordial  germ  cell 
(Fig.    17,   p.g.c).   although    at     this    time    there    are 


294        GERM-CELL   CYCLE   IN  ANIMALS 

eight  nuclei  in  the  egg.  As  determined  by  Kahle 
(1908)  and  confirmed  by  the  writer  (Hegner,  1912, 
1914a),  this  primordial  germ  cell  gives  rise  to  sixty- 
four  oogonia  and  to  no  other  cells.  This  is  the  nearest 
approach  to  a  complete  morphological  continuity 
of  the  germ  cells  that  has  yet  been  described,  and 
since  this  primordial  germ  cell  must  contain  the  germ- 
plasm  of  the  succeeding  generation,  the  condition 
in  this  fly  is  really  comparable  to  that  of  the  hypo- 
thetical case  cited  above,  only  in  Miastor  the  cell 
set  aside  for  reproductive  purposes  is  much  less  than 
one-half  of  the  egg,  the  somatic  part  of  the  egg  being 
not  a  single  cell,  but  a  syncytium  containing  seven 
nuclei. 

We  may  therefore  look  for  the  germ-plasm  of 
Miastor  in  the  primordial  germ  cell.  So  far  as  we 
know  there  are  only  two  sorts  of  materials  in  this 
cell,  that  contained  in  the  nucleus,  and  the  darkly 
staining  part  of  the  egg  which  becomes  recognizable 
just  before  maturation  occurs,  is  situated  at  the  pos- 
terior pole,  and  has  been  termed  the  pole-plasm 
(Fig.  13).  If  the  primordial  germ-cell  multiplies  by 
simple  division  and  if  there  is  an  equal  distribution 
of  the  contents  at  every  mitosis,  then  the  sixty- 
four  oogonia  must  each  possess  one  sixty-fourth  of 
both  the  nucleus  and  the  pole-plasm  of  the  primordial 
germ  cell  plus  any  materials  that  have  been  added 
during  the  period  of  multiplication.  An  enormous 
enlargement  occurs  during  the  growth  period  both  of 
the  nucleus  and  of  the  cell.  The  pole-plasm  cannot 
be    recognized    at    this    time,    but    again    becomes 


THE   GERM  PLASM   THEORY  ^2U5 

evident  just  before  nialuration  ;  it  lias  increased  in 
amount  to  approximately  sixty-four  limes  its  iornicr 
mass.  How  this  increase  lias  been  brou^'lit  about  is 
not  known,  but  it  has  been  su^^f^^esied  (p.  G8)  that 
preexisting  particles  of  ])()lc-plasni  may  grow  and 
divide,  or  the  dilution  of  the  j)ole-plasm  caused  l»y 
the  growth  of  the  egg  might  start  into  action  some 
catalyst  which  would  cause  the  production  of  niort* 
substance  Hke  the  pole-plasm  and  cease  its  activity 
when  the  amount  of  ])()le-plasm  characteristic  of 
the  mature  egg  had  accumulated  and  brought  it  to  a 
state  of  equilibrium.  In  the  midge,  C/iirunomu.s, 
the  primordial  germ  cell  is  segregated  even  earlier 
than  in  Miastor,  namely,  at  the  four-cell  stage. 
The  later  historv  of  the  germ  cells  is  not  so  well 
known  in  this  species,  however,  as  in  Minstor. 
The  data  presented  in  Chapters  V  and  \'I  ))i()\e 
that  a  definite  and  early  segregation  of  germ  ceils  is 
known  in  a  sufficient  number  of  groups  to  indicate 
that  the  process  is  quite  general  among  animals. 
The  morphological  continuity  of  the  germ  cells, 
how^ever,  cannot  be  established  with  such  a  degree  of 
certainty  in  the  vertebrates,  and  although  mu.^L 
investigators  believe  that  the  gvr\^^  cells  wvv  con- 
tinuous, still  the  entire  keind)ahn  has  never  been 
traced  as  accurately  as  it  has  in  many  invertebrates. 
Fortunately  almost  every  new  investigation  contains 
additional  data  and  more  refined  methods  which  lead 
us  to  hope  that  some  tini(^  in  the  nivir  future  the 
primordial  germ  cells  cvcmi  here  may  be  traced  back 
to  early  cleavage  stages. 


^296        GERM-CELL   CYCLE   IN  ANIMALS 

One  of  the  distinguishing  features  of  many  primor- 
dial germ  cells  is  the  presence  within  their  cytoplasm 
of  certain  stainable  bodies  to  which  I  have  applied 
the  term  "keimbahn-determinants."  Although,  as 
pointed  out  in  Chapter  VIII,  these  inclusions  do  not 
appear  to  consist  of  the  same  sort  of  material  in 
the  eggs  of  different  species  and  hence  their  signif- 
icance is  problematical,  still  they  seem  to  be  asso- 
ciated with  that  particular  part  of  the  egg  sub- 
stance which  becomes  the  cytoplasm  of  the  primor- 
dial germ  cells.  For  this  reason,  if  for  no  other, 
the  keimbahn-determinants  are  of  the  greatest 
value,  since  they  enable  us  to  determine  the  position 
of  this  germ-cell  substance  during  the  stages  before 
the  primordial  germ  cells  are  established.  It  is 
therefore  possible  to  trace  the  germ-cell  substance 
in  such  cases  as  Sagitta  (Fig.  54),  where  there  is  no 
morphological  continuity  of  the  germ  cells.  What 
relation  the  keimbahn-determinants  have  to  the  germ- 
plasm  is  not  yet  definitely  known. 

There  have,  of  course,  been  many  objections  to 
the  germ-plasm  theory.  The  history  of  the  germ 
cells  in  the  Coelenterata,  upon  which  Weismann 
(1882)  based  a  large  part  of  his  argument,  is  consid- 
ered by  Hargitt  (see  p.  95)  to  be  directly  opposed 
to  the  hypothesis.  According  to  some  zoologists 
there  is  no  essential  difference  between  the  repro- 
ductive cells  and  the  various  sorts  of  somatic  cells ; 
they  have  all  arisen  as  the  result  of  division  of  labor, 
and  the  germ  cells  have  been  differentiated  for  pur- 
poses of  heredity  just  as  the  muscle  cells  have  been 


THE   GERM-PLASM   THEORY  297 

differentiated  for  causing  motion  and  the  nerve  cells 
for  receiving  and  conducting  stimuli.  That  the  irerm 
cells  remain  in  a  primitive  condition  during  a  large 
part  of  the  embryonic  i)erio(l  is  accounted  for  hy 
the  fact  that  thev  become  functional  at  a  compara- 
tively  late  stage  in  ontogeny  (Eigenmann,  ISDO). 
Asexual  reproduction  by  means  of  fission  or  budding 
has  seemed  to  some  to  invalidate  the  theory  of  ger- 
minal continuity,  but  as  Montgomery  (IDOd,  ]>.  8''2) 
has  pointed  out,  "Perhaps  in  all  cases  products  of 
asexual  generation  contain  germ  cells.  If  this  were 
so,  it  might  then  be  the  case  that  the  incapacity 
of  any  part  of  the  body  of  an  animal  to  reproduce 
asexually,  or  even  to  regenerate,  would  l)e  due  to 
the  absence  of  germ  cells  in  it  —  but  this  is  merely 
a  suggestion."  The  probability  that  the  regenerat- 
ing pieces  of  coelenterates  and  the  artificial  Plas- 
modia formed  by  dissociated  sponge  cells  contain 
germ  cells  has  already  been  noted  (]).  79),  but  there 
are  cases  of  the  regeneration  of  sex  organs  that  are 
not  so  easily  explained.  For  example,  Janda  ( 191  ••2) 
has  found  that  if  the  anterior  part  of  the  hermaph- 
roditic annelid,  CriodrUus  laruinn,  is  n^moved,  a 
new  anterior  end  will  regenerate  containing  both 
ovaries  and  testes,  although  not  always  in  their 
normal  positions. 

The  study  of  the  germ  cells  in  the  cestode  Moniczia 
expansa  convinced  Child  (11)0())  that  germ  cells  may 
develop  from  tissue  cells.  In  this  species  the  germ  cells 
are  derived  from  the  parenchymal  syncytium,  which 
has  undergone  a  considerable  degree  of  cytoplasmic 


298        GERM-CELL   CYCLE   IN  ANIMALS 

differentiation  and  therefore  consists  of  real  tissue 
cells.  Those  parenchymal  cells  that  encounter 
certain  conditions  become  germ  cells.  Later  (1906) 
the  same  author  gave  an  account  of  the  development 
of  spermatogonia  in  the  same  animal  from  the  dif- 
ferentiated muscle  cells.  These  studies,  together  with 
the  results  from  experiments  on  regeneration,  have  led 
Child  (1912)  to  the  belief  ''that  this  germ-plasm 
hypothesis  and  the  subsidiary  hypotheses  which 
have  grown  up  about  it  are  not  only  unnecessary 
and  constitute  an  impediment  to  biological  thought, 
which  has  retarded  its  progress  in  recent  years  to  a 
very  appreciable  extent,  but  furthermore,  that  they 
are  not  in  full  accord  with  observed  facts  and  can 
be  maintained  only  so  long  as  we  ignore  the  facts." 
He  further  maintains  that  if  protoplasm  is  a  physico- 
chemical  substance  it  is  capable  of  changing  its  con- 
stitution in  any  direction  according  to  the  conditions 
imposed  upon  it,  and  that  therefore  the  continuous 
existence  of  a  germ-plasm  with  a  given  specific 
constitution  is  vmnecessary. 

The  evidence  in  favor  of  the  germ-plasm  theory 
is  so  strong  that  the  arguments  thus  far  advanced 
against  it  have  had  but  little  influence.  If,  then,  we 
accept  germinal  continuity  as  a  fact  and  consider 
the  germ-plasm  to  be  a  substance  that  is  not  con- 
taminated by  the  body  in  which  it  lies,  but  remains 
inviolate  generation  after  generation,  we  should  next 
inquire  as  to  the  nature  of  this  substance.  The 
generally  accepted  idea  is  that  the  chromatin  of  the 
nucleus  represents  the  physical  basis  of  heredity.     In 


THE   GERM-PLASM    IIIEORY  299 

favor  of  this  view  are  the  i'acts  thai  (Inriiiii-  mitosis 
the  number  and  shape  of  the  cliroinosonies  are  con- 
stant in  every  species  (variations  soinetiines  occur) 
and  the  complex  series  of  processes  in  indirect  nuclear 
division  seems  to  be  for  the  sole  purpose  of  dividing 
the  chromosomes  equally  between  the  daughter 
cells;  even  during  the  intervals  (interkinesis)  be- 
tween successive  mitoses  the  chromosomes  may  be 
recognized  in  certain  species  as  prochromosomes 
(see  Digby,  1914,  for  review  of  literature).  During 
the  maturation  of  the  germ  cells  chromosomes 
seem  to  play  the  most  important  role,  uniting  in 
synapsis,  and  separating  in  the  reducing  division. 
The  chromosomes  of  the  minute,  motile  spernui- 
tozoa  equal  in  number  those  of  the  comparatively 
enormous,  passive  egg;  the  spermatozoon  consists 
almost  entirely  of  chromatin,  and  this  is  tlu^  only 
substance  present  in  the  zygote  that  is  e(iually 
contributed  by  both  egg  and  spernuitozoiin.  The 
processes  following  the  penetration  of  the  spermato- 
zoon into  the  egg  bring  about  a  combination  of  the 
chromosomes  of  the  two  gametes  into  a  single 
nucleus;  in  certain  animals  at  least  some  characters 
depend  upon  the  presence  of  a  certain  chromosome, 
the  X-chromosome ;  in  certain  cases  of  ])olyspcrmy 
the  addition  of  extra  male  chromosomes  seems  to 
be  the  cause  of  tlu^  abnormal  dcn-c^lopnuMit  of  the  egg. 
These  and  many  other  facts  of  chromosome  be- 
havior that  have  been  discovered  by  observations 
and  experiments  have  convinced  most  biologists 
that  the  chromatin  is  the  germ-plasm. 


300        GERM-CELL   CYCLE   IN  ANIMALS 

It  is  becoming  more  and  more  evident,  however, 
that  the  cytoplasm  cannot  be  entirely  excluded.  As 
noted  in  Chapter  IX,  the  mitochondria  appear  to  be 
constant  cell  elements  and  may  actually  constitute 
a  part  of  the  essential  hereditary  substance.  Even 
if  these  particular  cytoplasmic  bodies  do  not  repre- 
sent germ-plasm,  still,  as  pointed  out  by  Guyer  (1911) 
and  others,  cytoplasm  as  well  as  nuclear  material  is 
necessary  to  explain  the  phenomena  which  we  call 
heredity.  It  was  shown  in  Chapter  I  that  the  most 
important  primary  constituents  of  protoplasm  are 
the  proteins,  and  the  idea  is  rapidly  becoming  general 
that  the  mechanism  of  heredity  consists  of  (1)  fun- 
damental species  substances,  probably  mainly  pro- 
tein in  nature,  together  w4th  (2)  equally  specific 
enzymic  substances  which  regulate  the  sequences  of 
the  various  chemical  and  physical  processes  incident 
to  development  (Guyer,  1911,  p.  299).  The  chro- 
mosomes have  been  suggested  as  enzymatic  in 
nature  (Montgomery,  1910),  but  enzymes  are  sup- 
posed merely  to  accelerate  reaction  already  initiated, 
and  hence  the  substrate  must  be  of  as  great  importance 
as  the  enzymes  which  work  upon  it.  But  the  sub- 
strates must  be  extremely  numerous  to  supply  each 
species  with  its  specific  proteins.  That  there  are 
enough  configurational  differences  in  corresponding 
protein  molecules  to  supply  the  number  for  the 
thousands  of  animal  species  is  certain,  since  some 
comparatively  simple  proteins  may  possess  thousands 
of  millions  of  stereoisomers.  Thus  the  study  of 
heredity  substance  involves  primarily   a  knowledge 


THE    GER:\I  PLASM    TIIKOKY  801 

of  the  nature  and  reactions  of  I  lie  clieinical  constitu- 
ents of  protoplasm,  for,  as  AVilsou  flDbJ,  p.  (Ui)  says, 
*'The  essential  conclusion  tluit  is  iudicatcd  l)\  cvto- 
logical  study  of  the  uuclear  substance  is,  thai  it  is  an 
aggregate  of  many  different  chemical  couipouents 
which  do  not  constitute  a  mere  mechanical  mixture, 
but  a  complex  organic  system,  and  which  inuhTgo 
perfectly  ordered  processes  of  segregation  and  dis- 
tribution in  the  cvcle  of  cell  life." 

Some  of  the  strongest  evidence  that  the  germ- 
plasm  must  include  cytoplasmic  constituents  is 
afforded  by  the  observations  and  experiments  ch^aling 
with  the  differentiation  of  the  germ  cells,  especially 
during  early  embryonic  development.  ^Fhe  writer's 
morphological  and  experimental  studies  of  chrysom- 
elid  beetles  seem  to  prove  that  the  nuclei  during 
the  cleavage  stages  are  all  potentially  alike  and  that 
it  is  the  cytoplasm  which  decides  their  fate.  Boveri's 
experiments  on  the  eggs  of  Ascaris  likewise  show 
that  the  cytoplasm  determines  the  initiation  of  the 
chromatin-diminution  process  and  controls  the  differ- 
entiation of  the  germ  cells.  Furthernu)re,  nuich  of 
the  data  in  the  preceding  cha])ters  indicates  that  the 
non-nuclear  substance  which  will  beconu'  segregated 
within  the  primordial  germ  cell  is  present  in  a  nu)re 
or  less  definite  region  in  the  nndivided  egg,  being 
gradually  localized  and  separated  from  the  other  v\i^;^ 
substances  as  cleavage  progresses.  'V\\v  })osili()n  of 
this  germ-cell  substance  can  in  many  ca.ses  be  (U'ter- 
mined  because  of  the  presence  ol  niclusions  ot  vari- 
ous sorts,  but  whether  these  keimbahn-determinants 


302        GERM-CELL   CYCLE   IN  ANIMALS 

constitute  an  important  part  of  the  germ-plasm  or 
play  a  minor  role  in  heredity  is  still  uncertain. 

Modern  cytological  studies  and  the  results  of  ex- 
perimental breeding  both  help  to  solve  the  prob- 
lems of  the  combination  and  subsequent  distribution 
of  the  determiners  or  factors  within  the  germ-plasm. 
In  fact,  it  has  been  maintained  by  certain  geneticists 
that  "The  modern  study  of  heredity  has  proven 
itself  to  be  an  instrument  even  more  subtle  in  the 
analysis  of  the  materials  of  the  germ  cells  than  actual 
observations  on  the  germ  cells  themselves  "  (Morgan, 
1913,  p.  v).  Those  who  do  not  wish  to  commit 
themselves  as  to  the  physical  or  chemical  nature  of 
the  germ-plasm  are  content  to  speak  of  determiners, 
factors,  or  genes  without  connecting  them  with  any 
particular  substances.  The  behavior  of  the  chro- 
mosomes, however,  enables  us  to  explain  so  many  of 
the  facts  of  heredity  that,  as  stated  above,  these 
bodies  are  generally  considered  to  constitute  the 
essential  hereditary  substance. 

The  study  of  heredity  was  wonderfully  stimulated 
by  the  recognition  in  1900  by  Correns,  Von  Tscher- 
mak,  and  de  Vries  of  the  results  of  Mendel's  (1866) 
investigations  on  plants.  One  of  the  simplest  of 
Mendel's  experiments  is  that  which  he  performed 
with  differently  colored  peas  (Fig.  81).  A  pea  bear- 
ing green  seeds  was  crossed  with  a  pea  bearing  yellow 
seeds.  The  first  (Fi)  generation  of  peas  resulting 
from  this  cross  all  bore  vellow  seeds.  When  the  in- 
dividual  plants  of  this  generation  were  inbred,  three- 
fourths  of  the  resulting  {F2)  generation  were  yellow 


THE   GERM -PLASM   THEORY 


303 


and  one-fourth  green.  This  proved  that  the  seeds 
of  the  first  generation  (Fi),  althougli  yellow,  si  ill 
possessed  within  them  the  factor  for  grtH'iiness  in  a 
latent  condition.     Green  was  therefore  called  a  re- 


Fz 


0 


F3 


Fig.  81. —  Diagram  to  illustrate  Mondel's  law  of  aogregution.  Individ- 
uals (zygotes)  are  represented  by  suptTiinposed  circles,  \vho.se  colors 
stand  for  the  factors  involved.  Gamctrs  (Kcrni  cells)  are  repreaeutcd 
by  single  circles.     {From  Morgan,  1U14-) 

cessive  character  and  yellow  a  dominant  character. 
As  a  result  of  breeding  the  (F2)  secumi  generation  it 
was  found  that  all  of  the  green  seeds  produced  plants 
which  bore  green  seeds;  that  is,  these  plants  were 
pure  green  and  "homozygous"  as  regards  color; 
whereas  the  plants  which  bore  yellow  seeds  couKl  l)e 


304         GERM-CELL   CYCLE   IN  ANIMALS 

separated  into  two  groups ;  one,  containing  on  the 
average  one-third  of  these  plants,  was  pure  yellow  and 
homozygous  as  regards  color ;  the  other  two-thirds, 
although  yellow,  contained  green  in  a  latent  condi- 
tion and  were  therefore  impure  yellows  and  "hetero- 
zygous" as  regards  color.  The  conclusion  reached 
was  that  the  eggs  and  spermatozoa  produced  by  the 
first  (Fi)  generation  (see  Fig.  81)  were  pure  yellow  or 
pure  green  and  that  chance  combinations  during 
fertilization  resulted  in  the  three  classes  of  individ- 
uals in  the  second  (F2)  generation ;  that  is,  one-fourth 
pure  yellow,  one-fourth  pure  green,  and  one-half  with 
dominant  yellow  and  green  recessive.  Evidently 
the  factors  for  yellow  and  green  repulsed  each  other 
during  the  maturation  so  that  they  became  localized 
in  different  germ  cells. 

Such  a  characteristic  as  the  color  of  the  seeds  of 
these  peas  is  kno^vn  as  a  unit  character,  and  the  sepa- 
ration of  the  factors  of  such  a  character  during 
maturation  is  referred  to  as  the  principle  of  segrega- 
tion, Mendel  further  discovered  that  if  the  seeds 
were  also  wrinkled  or  round,  such  characters  behaved 
independently  of  the  color  characters.  These  and 
other  experiments  described  by  Mendel  opened 
the  way  for  new  lines  of  investigation  which  have 
yielded  results  of  vast  importance  from  the  stand- 
point of  heredity  and  evolution.^ 

Soon  after  Mendel's  results  were  "rediscovered" 

^  For  more  detailed  accounts  of  experiments  and  theories  that  have 
been  pubhshed  wathin  the  past  fourteen  years  the  reader  is  referred 
to  the  books  of  Bateson  (1909,  1913)  and  Punnet  (1911). 


THE   GERM-PLASM   TIIEORV 


30^ 


it  was  pointed  out  by  Guycr  (100^2),  Sutloii  fl!)03), 
and  others  that  the  distribution  of  the  achill  cliar- 
acteristics  of  hybrids  which  were  found  by  Mendel 
to  reappear  in  the  offspring  in  rather  (k'finilc  propor- 


||.    |i.    \l'    \\    ]\'    J8- 

Fig.  82.  —  Diagrams  to  show  the  pairs  of  (  hroiiiDsoiurs  and  their  Ik»- 
havior  at  the  time  of  maturation  of  the  (•««.  Three  pairs  of  eliromo- 
somes  are  represented  ;  three  from  one  parent,  three  from  the  other. 
The  six  possible  modes  of  separation  of  these  three  are  shown  in 
the  lowest  line.      (From  Morgan,  WI4) 

tions,  could  be  explained  if  these  cliaraeteristies  are 
located  in  the  chromosomes.  During-  synapsis,  as 
already  explained  (p.  44),  homologous  maternal  and 
paternal  chromosomes  are  sui)posed  to  pair  and  then 
separate  in  the  reduction  division.  Tt  seems  ])robable 
that  the  pairs  of  chromosomes  do  not    occnpy  any 


306 


GERM-CELL  CYCLE  IN  ANIMALS 


definite  position  on  the  spindle  at  this  time,  but,  as 
indicated  in  Fig.  82,  the  distribution  of  the  maternal 
and  paternal  chromosomes  to  the  daughter  cells  is 
entirely  a  matter  of  chance.  If  the  homologous 
maternal  and  paternal  chromosomes  really  are  dis- 
tributed by  chance  to  the  eggs  and  spermatozoa 
following  synapsis,  then  the  number  of  combinations 
possible  are  as  follows  (Sutton,  1903)  : 


Somatic  Series 

Reduced  Series 

Combinations  in 
Gametes 

Combinations  in 
Zygotes 

2 

1 

2 

4 

4 

2 

4 

16 

8 

4 

16 

256 

16 

8 

'25Q 

65536 

24 

12 

4096 

16777216 

36 

18 

262144 

68719476736 

The  only  direct  evidence  that  such  distribution 
of  chromosomes  takes  place  is  that  furnished  re- 
cently by  Carothers  (1913)  from  a  study  of  the 
spermatogenesis  of  three  Orthopterous  insects, 
Brachystola  magna,  Arphia  simplex,  and  Dissosteira 
Carolina.  Miss  Carothers,  while  working  in  Pro- 
fessor McClung's  laboratory,  discovered  a  tetrad  in 
the  first  spermatocytes  of  these  insects  which  consists 
of  two  unequal  dyads  (Fig.  83).  During  the  two  mat- 
uration divisions  the  four  parts  of  this  tetrad  pass 
to  the  four  spermatozoa,  and  consequently  two  sorts 
of  spermatozoa  are  produced  so  far  as  this  chromo- 
some is  concerned,  one-half  with  one  of  the  larger 
elements  of  the  tetrad  and  one-half  with  one  of  the 


THE    (;i:R.M  PLASM    TIIKORY  307 

smaller  elements.  These  diU'ereiillv  .sized  (1\  .ids  an- 
considered  by  Carotliors  as  "distinct  pliy.siolo^n'cal 
individuals,  representing^  resj)ecli\  <*ly  I  he  j)at('riial 
and  maternal  contribution  to  the  formation  of  some 
character  or  characters;  and,  as  each  can  he  iden- 
tified, they  furnish  an  excellent  means  of  tracing  tlie 
process  of  segregation  and  recombination  ''  (p.  499). 
It  was  at  first  assumed  that  eacli  of  tlie  pairs  of 
chromosomes  which  unite  in  synapsis  was  respon.sible 
for  a  single  adult  ^ 

character,  birt    ^ U 

the  number  of     d A  /^/"^^i^O*^ 

Mendelian    char-    ^rp  0%  Q  P9  VAAj^ 

acters    is    known  ^ 

,         I  ,  •         Fig.  83.  —  Arjjftia  simplex.     Chromosomes  ol 

LO  oe  greater  m  ^^.g^  spermatocyte,  o  =  accessory  chromo- 
Certain  cases  than         '^o"'^  •     ^  =  unequal  dyad.    {Frum  Caruthers, 

the  number  ot 

chromosomes.  Fortunately,  it  has  been  f()nn<l  that 
the  characters,  instead  of  unch'rgoing  iuih'peiKh'ut  as- 
sortment, mav  become  hnked  so  that  certain  of  them 
almost  always  occur  together  in  the  oil*sj)ring.  The 
relation  of  these  facts  to  the  constitution  of  the 
chromosomes  mav  best  be  il  hist  rated  bv  reference  to 
the  studies  of  Morgan  and  his  stu(h'nts  on  the  fruit - 
fly,  DrosopJiila.  Over  one  hundred  nuitants  of  tliis 
species  have  been  discovered  by  these  in\estigators. 
So  far  as  studied,  the  characters  of  lliese  Hies  seem  to 
form  three  groups.  "The  characters  in  t  he  first  gronj) 
show  sex-linked  inheritance.  They  follow  the  sex- 
chromosomes.  The  second  group  is  less  extensive. 
Since  the  characters  in  this  group  arc  linked  to  each 


308       GERM-CELL   CYCLE   IN   ANIMALS 

other,  we  say  that  they  He  in  a  second  chromosome. 
The  characters  of  the  third  group  have  not  as  yet 
been  so  fully  studied,  except  to  show  that  they  are 
linked.  We  place  them  in  the  third  chromosome 
without  any  pretensions  as  to  which  of  the  pairs  of 
chromosomes  are  numbered  II  and  III. 

*'  The  arrangement  of  these  characters  in  groups 
is  based  on  a  general  fact  in  regard  to  their  behavior 
in  heredity,  viz.,  A  member  of  any  group  shoivs  linkage 
with  all  other  members  of  that  group,  but  shows  inde- 
pendent assortment  with  any  member  of  any  other 
group.'*  If  the  factors  which  determine  these  groups 
of  characters  are  situated  in  the  chromosomes,  as  the 
hypothesis  demands,  we  should  expect  each  group 
to  act  as  a  unit  in  heredity.  Occasionally,  however, 
the  characters  of  a  group  appear  to  act  independently, 
and  there  must  thus  be  an  interchange  of  factors  at 
the  time  of  synapsis.  As  already  stated  (p.  254),  an 
interchange  of  substances  between  chromosome  pairs 
during  synapsis  is  possible  and  even  probable.  Mor- 
gan explains  the  degree  of  crossing  over  of  characters 
in  the  following  way :  The  factors  which  determine 
the  characters  are  arranged  in  the  chromosomes  in 
a  linear  series ;  those  factors  that  are  near  together 
will  have  less  chance  of  being  separated  than  those 
that  lie  farther  apart.  The  relative  distances  be- 
tween these  factors  can  be  judged  by  the  frequency 
of  interchange  as  determined  by  breeding  experi- 
ments. It  has  thus  been  possible  to  locate  certain 
factors  in  the  chromosomes  more  or  less  accurately 
and  to  predict  with  some  degree  of  certainty  the  re- 


THE   GERM-PLASM   TIIKOin  309 

suits  of  hybridization.  Tims  If  \]\v  j)()siti()n  of  a 
newly  discovered  factor  is  deteniiiiicd  1)\  compariMjii 
with  another  particular  known  factor,  it  i\  pos^flijc 
to  ''calculate  the  results  for  all  other  known  factors 
in  the  same  chromosome."  ]\Ior<(an's  ideas  re^'ard- 
ing  the  organization  of  the  chromosomes  coincide 
with  those  expressed  hy  AVeismann  in  one  r<'spect, 
that  is,  they  are  assumed  ''to  have  definite  strnctnres 
and  not  to  be  simply  bags  filled  with  a  hoino^^^eneons 
fluid."  Wilson  (191^2,  p.  (),S)  also  regards  the  chro- 
mosomes as  "componnd  })o(Iies,  consisting  of  diller- 
ent  constituents  which  undergo  dilferenl  modes  of 
segregation  in  different  species." 

Students  of  genetics  now  consider  the  iinhvidnal 
as  built  up  of  a  number  of  unit  characters  represented 
in  the  germ-plasm  by  factors,  and  when  two  different 
germ-plasms  unite  (amphimixis)  the  factors  do  not 
mix,  but  remain  uncontaminated.  The  germ-plasm 
of  offspring  which  develop  from  fertilized  vfi^i^s  is 
supposed  to  consist  of  an  assortment  of  factors 
brought  about  during  synapsis  and  rednctionas  indi- 
cated in  Fig.  84.  The  factors  (or  genes)  in  the  germ- 
plasm  occur  in  pairs  called  alleloni()ri)hs,'  and  one  of 
the  pair  may  be  regarded  as  donn'nant,  the  other  re- 
cessive, as,  for  exani})le,  the  yellow  and  green  color>  of 
pea  seeds.  Thns  the  appearance  of  the  individual 
depends  upon  the  character  of  its  dominant  factors. 
Any  attempt  to  account  for  the  origin  of  new  species 

1  According  to  some  investigators,  especially  in  Kngland.  tlie  pn'senct* 
of  a  factor  should  be  considered  one  ailelonuirph  and  its  absence  jus  the 

contrasting  factor. 


310        GERM-CELL   CYCLE   IN  ANIMALS 


Fig.  84. —  Diagrams  illustrating  the  union  of  two  stocks  with  paired 
factors  A,  B,  C,  D,  and  a,  b,  c,  d,  to  form  pairs  Aa,  Bb,  Cc,  Dd. 
Their  possible  recombinations  are  shown  in  the  sixteen  smaller 
circles.     {After  Wilson.) 

must  accept  these  facts  of  heredity  as  a  basis.  If 
evolution  is  a  fact,  new  species  must  have  arisen  from 
time  to  time.  This  may  have  occurred  by  the  drop- 
ping out  of  old  factors  or  the  addition  of  new  factors. 
There  seems  to  be  sufficient  evidence  that  factors 
are  sometimes  left  out,  but  there  are  very  few  cases 
of  the  addition  of  new  factors.  Our  ideas  of  a  pro- 
gressive evolution  demand  the  addition  of  new  factors, 
but  whether  this  is  brought  about  by  changes  within 
the  germ-plasm  or  is  the  result  of  external  influences 
is  not  known. 

D.  H.  HILL  LIBRARY 

North  C^rc^'i--  "•-'-  -"nlUne 


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INDEX  OF  AUTHORS 

All  numbers  refer  to  pages.  An  asterisk  (*)  after  a  pnur  niinilMT  in- 
dicates that  the  title  of  a  contribution  by  the  author  will  U-  fmiiul  uu 
that  page. 


Allen,  B.  M.,  32, 100, 102,  206,  311.* 
Altmann,  276,  282,  311.* 
Amma,  140,  163/.,  216,  228,  311.* 
Ancel,  195-197,  311.* 

Baer,  van,  192. 

Balbiani,  107,  214,  229,  311.* 

Baltzer,  261. 

Bambeke,  van,  222,  311.* 

Bancroft,  21. 

Bardeleben,  273,  311.* 

Bartelmez,  232,  311.* 

Bateson,  312.* 

Beard,  100,  312.* 

Beckwith,  135,  312.* 

Benda,  40,  276,  277,  279,  282,  312.* 

Beneden,  van,  80,  82,  87,  88. 

Berenberg-Gossler,    98,    100,    102, 

312.* 
Bessels,  118,  312.* 
Bigelow,  172,  186,  225,  312.* 
Blockmann,  185,  221,  225,  312.* 
Bonnevie,  177,  286,  313.* 
Bouin,  206,  276,  313.* 
Boveri,  174  /.,  184,  193,  195,  217, 

230,  268,  301,  313.* 
Brandt,  118,  313.* 
Brauer,  83,  313.* 
Brown,  3. 
Browne,  285,  313.* 
Buchner,  123,   140,  180,  187,  195, 

222,  286,  313.* 
Bunting,  87,  314.* 
Buresch,  195,  199/.,  226,  314.* 


Calkins,  26,  314.* 

Carothers,  306,  307,  314.* 

Carter,  74,  314.* 

Castle,  192,  314.* 

Caulleiy,  195.  314.* 

Champy,  195,  206,  209,  3U.» 

Child,  130,  188,  297,  314.* 

Chun,  184,  315.* 

Cohn,  3. 

Cole,  207,  209.  315.* 

Conklin,  218,  232.  23.3.  234.  315.* 

Correns,  302,  316.* 

Cowdry,  280.  285,  315.* 

Cunningham,  20!).  315.* 

Debaisieux.  120.  121.  122.  315.* 

Delage,  195.  315.* 

Delia  Vallo,  12.315.* 

DemoU,  195.  202/..  316. 

Desii,  70,  76,  316.* 

DickrI,  144,  316.* 

I)i^M)y,  299,  316.* 

Dobell,  28,  316.* 

Dodds.  102-103.  ;U(5.* 

Dontaster,  275.  316.* 

Downing.  83-85,  97.  188.  316.* 

Drie.sch.  161.  231.  316* 

Duesberg,   104.  23:{.  273.  2S0.  tsX 

31(i.* 
Dujardiii,  3. 
Diirmc,  van.  286.  316.* 
Dustin.  99.  206.  317.* 


KinvnlM-rg.  S2.  317.* 
Kigenmanii.  100.  243.  297.  317. 


••  • 


Z 


337 


338 


INDEX  OF  AUTHORS 


Elpatiewsky,  26,  140,  179  /.,  195, 

228,  317.* 
Escherich,  107,  317.* 
Evans,  75,  317.* 

Farmer,  273. 

Faure-Fremiet,  13,  279,  283,  285, 

317.* 
Feistmantel,  207. 
Felt,  52,  317.* 
Fiedler,  73,  317.* 
Firket,  99,  317.* 
Fischer,  12. 

Flemming,  214,  273,  318.* 
Fol,  186,  318.* 
Foot,  123,  137,  214,  318.* 
Friedmann,  207,  318.* 
Frischholz,  97,  318.* 
Fuchs,  163,  169,  318.* 
Fujita,  186,  225,  318.* 
Fuss,  100,  318.* 

Galton,  290,  318.* 

Gardiner,  157,  318.* 

Gates,  160,  318.* 

Gerhartz,  207,  318.* 

Giardina,  120-122,  223,  231,  318.* 

Giglios-Tos,  279,  319.* 

Goette,  75,  95,  319.* 

Goldschmidt,  222,  279,  286,  319.* 

Gorich,  73,  319.* 

Govaerts,  120,  123,  128,  319.* 

Graber,  107,  319.* 

Granata,  279. 

Grimm,  107,  310.* 

Grobben,  163,  170,  319.* 

Gross,  137,  319.* 

Gudematsch,  194,  319.* 

Guenther,  83,  319.* 

Gunthert,  121,  122,  128,  319.* 

Guilliermond,  277,  319.* 

Gutherz,  273,  320.* 

Guyer,  272,  300,  305,  320.* 

Hadzi,  83,  320.* 
Haeckel,  320.* 


Haecker,  36,  73,  12'4,  140,  163  /., 

184,  215,  320.* 
Hallez,  112,  320.* 

Hargitt,  C.  W.,  86,  88,  95,  296,  320.* 
Hargitt,  G.  T.,  96,  320.* 
Harm,  88,  89,  98,  320.* 
Harman,  136,  320.* 
Harmer,  161,  321.* 
Hartmann,  216,  321.* 
Harvey,  1. 
Hasper,  104,  107, 110,  140,  218,  230, 

235,  321.* 
Hegner,  33,  51,  107,  140,  219,  225, 

235,  286,  294,  321.* 
Heider,  79. 

Henking,  106,  256,  321.* 
Herbst,  216,  321.* 
Herold,  118,  321.* 
Herrick,  215,  322.* 
Hertwig,  O.,  82,  231,  322.* 
Hertwig,  R.,  83,  222,  322.* 
Heymons,  186,  194,  322.* 
His,  231. 

Hodge,  214,  322.* 
Hogue,  179,  322.* 
Holmes,  137,  322.* 
Hooke,  2,  207,  322.* 
Hoven,  277,  280,  322.* 

Ijima,  76. 

Ischikawa,  86,  163,  170,  322.* 

Jager,  291,  322.* 
Janda,  297,  322.* 
Janssens,  254,  255,  322.* 
Jarvis,  100,  322.* 
Jenkinson,  50,  232,  323.* 
Jennings,  186,  225,  323.* 
Jordan,  214,  288,  323.* 
Jorgensen,  72,  74,  78,  323.* 

Kahle,  51,  107,  140,  230,  235,  294, 

323.* 
Kellicott,  50,  323.* 
Kellogg,  118,  323.* 
King,  195,  206,  208,  323.* 


INDEX   OF  AUTHORS 


339 


Kite,  6,  323.* 

Kleinenberg,  80,  82,  83,  161,  323.* 

Knappe,  208,  323.* 

Kolliker,  373. 

Koltzoff,  323.* 

Korotneff,  83,  323.* 

Korschelt,  79,  324.* 

Kossel,  8. 

Kowalevsky,  107,  324.* 

Kruger,  195,  267,  324.* 

Kiihn,  140,  163/.,  225,  236,  324.* 

Kulesch,  105,  324.* 

Kunstler,  276,  324.* 

Kuschakewitsch,  100, 195, 206, 324.* 

Lams,  286,  324.* 

Lang,  157,  324.* 

La  Valette  St.  George,  207, 276, 324.* 

Lecaillon,  109,  111,  324.* 

Lewis,  281,  324.* 

Leuckart,  51,  107,  324.* 

Levene,  11. 

Leydig,  82,  325.* 

Lieberkiihn,  73,  325.* 

Lillie,  188,  232,  234,  325.* 

Loeb,  13,  21,  325.* 

Loewenthal,  222,  325.* 

Loyez,  286,  325.* 

Lubarsch,  130,  325.* 

Lubosch,  214,  325.* 

Maas,  73,  76,  78,  325.* 
McClendon,  172,  185,  325.* 
McClung,  256,  325.* 
McGregor,  134,  135,  326.* 
Malpighi,  3. 
Mangan,  187,  326.* 
Marchal,  161,  326.* 
Marcora,  280,  326.* 
Marshall,  A,  326.* 
Marshall,  W.,  75,  326.* 
Marshall,  W.  M.,  222,  326.* 
Maupas,  267,  326.* 
Mayer,  283. 
Megusar,  113,  326.* 
Meinert,  51,  326.* 


Mendel,  .302/.,  .326.* 
Metschnikolf.  51,  107,  1H.3,  221.  .S2(J.» 
Moves,  134,  216.  244.  200,  2K4.  2H7. 

326.* 
Meyer.  170.  327.* 
Mimiiin,  70,  72.  327.* 
Mohl,  von,  3. 
Montgomery,    129.    l.U,    195,   214. 

241,  285.  297,  .300,  327.* 
Moore,  273,  328.* 
Morgan,    192,   232,   255.   2(55,   3<)2. 

307,  309,  328.* 
Morse,  1.37,  244,  328.* 
Muller,  F.,  1J)5.  328.* 
Muller,  K..  77,  80,  .328.* 
Muller-Cale.  172,  328.* 
Mulsow.  256,  257. 
Munson,  226,  329.* 

Nachtsheim,143, 145,  205,  200.;{^t».' 
Xoack,  107,  109.  Ill,  225.  2,35.  329.* 
Nussbamn,  83,  100,  291,  329.' 

Ognew,  208,  329.* 
Okkeberg,  209,  329.* 
Ostwald,  9. 
Owen,  290,  329.* 

Patterson,  157,  161.  .329.* 
Pauleke,  120,  122,  222,  329.* 
Paulmier,  250.  329.* 
Payne,  138,  .329.* 
Pei-seneer,  195,  329.* 
Petrunkewitseh.  143.  145.  205,  3.3<».* 
PBiiger,  205.  231. 
Pick.  194,  .3.30.* 
Prenant,  279,  2S3.  .330.* 
Preusse.  137,  330.* 
Punnet,  330.* 

Rath,  v(.m.  134,  i:i5.  3.30.* 

Regaud,  279,  282.  330.* 

Rhode.  218.  ,330.* 

Richards,  1.3(5.  3.30.* 

Ritter.  107,  lOS.  22!>.  235.  ;U0.' 

RolxTtson.   101.  .3.3(1.* 

Robin,  107,  330.* 


340 


INDEX   OF  AUTHORS 


Rosel  V.  Rosenhoff,  72,  331.* 

Rosner,  161,  331.* 

Roux,  141. 

Rubaschkin,  98,  100,  103,  226,  283, 

331.* 
Ruckert,  99,  331.* 
Russo,  286,  331.* 

Samassa,  170,  172,  331.* 

Sauerbeck,  194,  331.* 

Schapitz,  100,  331.* 

Schaxel,  214,  331.* 

Schaffer,  283. 

Schleip,  195,  268,  331.* 

Schleiden,  3. 

Schmidt-Marcel,  205,  207,  331.* 

Schmiedeberg,  12. 

Schneider,  83,  331.* 

Schonemund,  194,  331.* 

Schonfeld,  237,  331.* 

Schreiner,  209,  332.* 

Schulze,  76,  80,  193,  332.* 

Schwann,  3. 

Selenka,  157,  332.* 

Semon,  206,  332.* 

Siebolt,  von,  193,  332.* 

Silvestri,  143,  145,  215,  332.* 

Simon,  194,  332.* 

Smallwood,  87,  98,  332.* 

Spooner,  234. 

Steudel,  12. 

Stevens,  140,180, 195,228, 256,  332.* 

Strasburger,  214,  332.* 

Stricht,  van  der,  188,  286,  333.* 

Strobell,  123,  137,  214. 

Stuhlmann,  221,  333.* 

Suckow,  118,  333.* 

Surface,  157,  333.* 

Sutton,  305,  306,  333.* 

Swarezewsky,  26,  333.* 

Swift,  33,  103,  226,  333.* 

Tannreuther,  83,  333.* 
Tennent,  261. 
Thallowitz,  88,  333.* 
Trembley,  82,  333.* 


Tschaschkin,  98,  102,  226, 302, 333.* 
Tschermak,  333.* 

Uffreduzzi,  194,  333.* 

Vander  Stricht,  187,  333.* 
Varenne,  82,  333.* 
Vejdovsky,  334.* 
Voeltzkow,  107,  334.* 
Vollmer,  172,  334.* 
Voss,  von,  204,  334.* 
Vries,  de,  302,  334.* 

Wager.  83,  334.* 

Wagner,  51,  334.* 

Waldeyer,  98,  130,  334.* 

Walker,  159,  334.* 

WassiUeff,  286. 

Weismann,  25,  82,  88,  97,  107,  113, 

144,  296,  309,  334.* 
Weltner,  73,  75,  77,  334.* 
Wheeler,    33,    100,    109,    144,    157, 

185,  193,  335.* 
Whitman,  231,  335.* 
Wieman,  124,  138,  225,  273,  335.* 
Wierzejski,  75,  186,  225,  335.* 
Wijhe,  van,  99,  335.* 
Wilcox,  273,  335.* 
Wildman,  279,  335.* 
Wilke,  285,  336.* 
Wilson,  E.  B.,  4,  21,  133,  224,  232, 

250,  301,  309.  336.* 
Wilson,  H.  v.,  75,  77,  80,  336.* 
Winiwarter,  129, 132, 251,  273, 336.* 
Winter,  de,  119,  120. 
Woods,  100,  336.* 
Wolff,  2. 
Wulfert,  89,  98,  336.* 

Youngman,  207. 
Yung,  207,  336.* 

Zamik,  195,  269,  336.* 
Zeigler,  237. 
Zeleny,  232,  336.* 
Zoja,  287,  336.* 
Zykoff,  75,  336.* 


INDEX   OF  SUBJECTS 

All  numbers  refer  to  pages.  Words  in  iUilics  are  names  <.f  famili«-s. 
genera,  species,  or  of  higher  divisions.  Xunibers  in  thick  tyiH*  are  num- 
bers of  pages  on  which  there  are  figures. 


Aborting  spindle,  157. 

Accessory  chromosome,  134,  202. 

Acidophile,  11. 

Actinospherium,  222. 

Ageniaspis,  146. 

Allelomorph,  309. 

Alternation  of  generations,  23. 

Alveolar  structure  of  protoplasm, 
4. 

Amoebocyte,  71,  73,  79. 

Amia,  32,  33. 

Amitosis,  13-14,  133-139,  250. 

Amphiaster,  15. 

Amphibia,  amitosis,  134-135;  her- 
maphroditism, 205  ff. 

Amphimixis,  309. 

Amphiuma,  135. 

Amyloplastid,  7. 

Anaphase,  15,  16. 

Anello  cromatico,  121,  123,  223. 

Animal  pole,  20. 

Aptera,  life  cycle,  22. 

Arcella,  26. 

Archaeocyte,  70-73. 

Archoplasm,  5,  7. 

Arenicola,  188. 

Armadillo,  polyembryony  in,  161. 

Arphia,  307. 

Arfhropoda,  212. 

Ascaris,  122,  174  /.,  217  /.,  230, 
241,  301 ;  maturation  in,  2G1, 
263  ;    mitochondria  in,  284. 

Asexual  larvae,  149. 

Asplanchna,  186,  225. 


Aster,  16. 

Asterias,  6. 

Attraction-sj)here,  5,  7,  ii7. 

Amelia,  183. 

Aussenkornelu-n.  164.  213.  210.  228. 

Axolotl,  159,  208. 

Bacteria,  4,  186-187. 

Basophile,  1 1 . 

Bat,  188. 

Besondere    Kiirpcr.    180.    isj     /T . 

213,  228,  23!). 
Bidder's  organ,  207. 
Binary  fission.  17. 
Binuclearity  hy|)othesi.s,  27. 
Bioblast,  276. 
Bivalent  chrDmosomes.  44. 
lilastotomy,  161. 
Bryozoa,  161. 
Budding,  17,  22,  23.  69.  161,  297. 

Calligrapinu  109.  111.  2.30. 

Calliphora,  107.  111/..  235. 

Camponotus,  ^-^l . 

Canfliocamptu.t,  165. 

Cat,  187. 

Cell,  2-16;   definition.  3;   divisit»n. 

13-16;     lineage,    29;     shapr.    4; 

size.  4;    tiieory,  3. 
(  entrifuged  egg.s,  173. 
C'entro.sonu'.  6,  7.  14,  15.  164.  169. 

237,  238. 
Cerchratiihis,  232. 
Ct'stoiUi.  \'M\   137. 


341 


342 


INDEX   OF  SUBJECTS 


Chcetognatha,  212. 

Characters,  dominant,  303 ;  linked, 
307 ;  recessive,  303 ;  unit  303. 

Chiasmatype  theory,  254. 

Chick,  33^  100,  103,  227,  281. 

Chironomus,  108-109,  110,  224,  229, 
235. 

Chloroplastid,  7. 

Cholesterin,  8,  12,  13. 

Chorion,  113. 

Chondriodierese,  279. 

Chondriokont,  279. 

Chondriosome,  7,  102,  103,  168, 
227  /.,  275,  277. 

Chondriotaxis,  279. 

Chromatin,  5,  7,  11-12;  as  germ- 
plasm,  299 ;  as  keimbahn-deter- 
minants,  211  ^. 

Chromatin-diminution,  47,  56,  57, 
139-141,  174/.,  217/.,  249. 

Chromatin-nucleolus,  5,  7. 

Chromidia,  26,  123,  168,  221  /., 
279. 

Chromidial  net,  26. 

Chromosome,  6,  7,  14,  15,  243,  299 ; 
accessory,  106 ;  cycle,  245-275 ; 
diploid,  43;  division,  248;  in 
fertilization,  49;  haploid,  43; 
individuality,  255 ;  in  man, 
272  /. ;  and  Mendelism,  305 ; 
number,  246;  from  nucleolus, 
214 ;  in  parthenogenesis,  246 ; 
univalent,  249. 

Chrysemys,  32. 

ChrysomelidcB,  109. 

Ciona,  192. 

Cladocera,  163/. 

Clathrina,  70. 

Clava,  88,  135. 

Cleavage,  29,  115. 

Cockroach,  194. 

Coelenterata,  80-98,  212. 

Coleoptera,  109-143. 

Colloid,  9. 

Colony,  17. 


Compsiliira,  107,  109. 
Conjugation,  17. 
Copepoda,  165/ 
Copidosoma,  146/ 
Copulationszelle,  163. 
Corps  enigmatique,  187. 
Crepidula,  218. 
Crustacea,  163-173. 
Crystalloid,  9. 

Cyclops,  124,  164/,  228,  247. 
Cyviatog  aster,  100. 
Cynthia,  233,  280. 
Cyst  formation,  125-129. 
Cytomicrosome,  276. 
Cytoplasm,   6,    143,    179,    224   /., 
300/ 

Daphnidce,  163. 
Death,  natural,  25. 
Determination  of  sex,  118. 
Determiner,  302. 
Diaptomus,  165. 
Differentiation,  76,  141-143. 
Dioecious,  18,  190. 
Diploid,  chromosomes,  248. 
Diplotene,  252. 
Diptera,  107. 
Dispermic,  177,  178. 
Dominance,  303. 
Dotterplatte,  109,  115,  225,  235. 
Drosophila,  307/ 
Dyad,  45,  46,  306,  307. 
Dytisciis,  120-124,  121,  223. 
Dzierzon  theory,  143. 

Earthworm,  161,  190,  191. 

Eclectosome,  279. 

Ectosome,  166,  167  ff.,  213,  237. 

Egg,  19,  20. 

Encyrtus,  145. 

Enzyme,  300. 

Ephydatia,  75. 

Epigenesis,  2,  243. 

Ergastoplasma,  276. 

Eudendrium,  86. 


IXDKX    OF   SLIUKCTS 


HS 


Euschisiis,  281. 
Evolution,  310. 

Factor,  302,  309. 
Female  sex,  18. 

Fertilization,  44,  47-49,  2oG,  /. 
Fission,  22. 

Frog,  hermaphroditism  in.  20.5  (f. 
Fusion,   of  chromosomes,   234;    (tf 
oocytes,  152.  1.5.)^. 

Gel,  5,  6,  9. 

Gemmule,  18,  74-75,  76,  79. 

Genes,  302,  309,  310. 

Genetics,  309. 

Genetic-continuity  of  chromosomes. 

255. 
Germ  cell,  19-22,  101,  v.s.  somatic 

cell,  290-297. 
Germ-cell  cycle,  28-49. 
Germinal  continuity.  292. 
Germinal  epithelium  the(»ry.  98. 
Germinal  localization,  231. 
Germinal  spot,  214. 
Germinal  vesicle,  19,  20,  54. 
Germ-plasm,    in    A.scariji,    177,    in 

Hijdra,  8.3-85  ;  in  Miastnr,  29.3  ;  in 

polyembryony,   102;  in  sponj^rs. 

80. 
Germ-plasm  theory.  290-310. 
Gonochorism,  18,  191. 
Gonocyte,  71,  73. 
Gonothyrcpa,  89. 
Gonotome  theory,  97. 
Grafdla,  157. 
Gryllus,  123,  244. 
Guinea-pig,  102.  103.  104.  227. 
Gynandromorj)h,  1!>3   19i. 

Haploid,  247. 
Hauptnudeolus,  214. 
Helix,  195.  190/..  220. 
Hemipfcra,  amitosis  in.  137. 
Hermaphrodite.  18.  189-210.  20!). 
Ilclcrucopc,  105. 


n«'teroty|)ic  mitosis,  40.  252.  f.'i.'i. 

Ilrterozygou.s,  iJOi. 

Ilonuilogou.s  chroinoHomrA,  4J3, 

Hornotypic  mittwis,  40. 

Iloinozyguus.  ;{<»;{. 

Iloncy-lM-i'.  14.3   I  Vl.  2r,i /..  266. 

Hyaloplasm,  4.  6. 

Ili/dra.  82-85.  159. 

Ili/ilrartinia,  87. 

Hy<lroid.  lif.-  cyclr  at.  23. 

Illfdrophilus,  \  13. 

Ilydmzoa,  85-98. 

Hymcnopieru,  14.3-103,  221.  235. 


chrumosomrs. 


IdiochroMi.itin.  28. 
lndi\'i(lualit>-       of 

255. 
Interkinesis,  299. 
Isotropism,  231. 

Jellv-fish.  23. 


KarviK'hondria,  279. 
Karyokintvsis,  1.3.  1 1.  15 
Kar\dlyinpli.  (!. 
Karyosomr.  6.  7.  213. 
Kciiuhalm.   in  .tji/uorf,!,    ]s:\.    184, 

Aiiipfiihia,    200   Jf.;       (  hidoccra, 

103     /. ;       Coprpixlii.     105     ff.\ 

insiM-t-s.      100-103;       neinat«Hli*«. 

174    17!»;    Sagilla.  179/. 
Keinil».din-drt<Tininants.    19.    211- 

2U.  2!»«i.  .301  ;    ^;.u.->i>.  211    234; 

l.xalization.  2;U  2KI;    futr.  240- 

244. 
K«-iinl»ahn(hn»midirn.  22.3 
K«'irni)ahniliron»atiii.  152/.  223. 
KrimLalmplasina.     108.     110.     115. 

23(1.  2:U. 
K«-iml>ahn7.rlle.  1(>1. 
Krimlh-ik.  2U. 
K.-iinhautl.lastrm.  113.  lU 
Kriinsljil ti".  95. 
K.imwtiUt.  108.  110.   11.'..  ^^3.'. 
Keimzonc,  95. 


344 


INDEX  OF  SUBJECTS 


Kinetochromidia,  214. 
Kinoplasm,  214. 

Lamprey,  100,  209. 

Larva,  23. 

Lecithin,  8,  12. 

Lejpas,  172,  225. 

Lepidoptera,  118. 

Lepidosteus,  32,  33,  101. 

Leptinotarsa,  37-41,   111,  125-129, 

138-139. 
Leptotene,  251,  252. 
Life  cycles,  22^. 
Linin,  5,  7. 

Linked  characters,  307. 
Locust,  23. 
Lophius,  102. 
LygoBus,  259. 
Lymncea,  192. 

Macrogamete,  27. 

Male,  18. 

Man,    chromosomes    of,     272    ff. ; 

hermaphroditism  in,  194. 
Maturation,  41-47,  129,  256  #. 
Medusa,  23. 
Mesostoma,  204. 
Metabolism,    and    sex,    275 ;     and 

Keimbahn-determinants,  228. 
Metagenesis,  23. 
Metanucleolus,  183,  215. 
Metaplasm,  5,  7,  8. 
Metaphase,  15,  16. 
Metazoa,  1,  18. 
Miastor,  51-68,   107,  217  /.,  235, 

293-294. 
Microgamete,  27. 
Microsome,  6. 

Middle  piece,  of  sperm,  21,  216. 
Migration,    of    germ    cells,    31-34, 

101-102,  116,  226. 
Mitochondria,  5,  13,  39,  40,  226/., 

275-289 ;      methods,      282-283 ; 

Ascaris,  284;    chick,  278;    di^ii- 

sion  of,  281,    284 ;    function  of. 


286  /. ;    in  living  cells,  280,  281 ; 

in  plants,  277,  280 ;  reduction  of, 

285;   and  sex,  285. 
Mitosis,  13,  14-16. 
Mitrocoma,  183. 
Mixochromosomes,  251. 
Moina,  163. 
Mollusk,  185,  191. 
Monad,  chromosome,  45,  46. 
Moniezia,  136,  297. 
Monoecious,  18,  191. 
Monospermy,  48. 
Mosaic  development,  233. 
Moulting,  23. 
Musca,  107. 
Myofibril,  280. 
Myxine,  209. 
Myzostoma,  37,  185,  193. 

Nahrzellenkem,  170. 

Nebenkem,  203,  221,  285. 

Nebennucleolus,  214. 

Nematodes,  chromosomes  of,  267  ff. 

Nepa,  137. 

Neratina,  186,  225. 

Netzapparat,  103,  104. 

Neurofibril,  280. 

Nuclear  sap,  6. 

Nucleic  acid,  11. 

Nuclein,  11. 

Nucleolo  of  Silvestri,  145  ff. 

Nucleolus,  5,  6,  13,  167,  213/. 

Nucleoprotein,  8,  11. 

Nucleus,  3,  13-16. 

Nurse  cells,  35-36,  53,  119-121, 150, 

151,  201,  202. 
Nutritive  substances,  225  ff. 

(Enothera,  160. 
Oncopeltus,  261,  262. 
Oocyte,  38,  39,  40-41. 
Oogenesis,  42,  256  /. 
Oopfhora,  145,  146. 
Ophryotrocha,  37. 
Opossum,  288. 


IXDKX    OF   SUBJECTS 


345 


Organ-forniinp  substances,  233. 
Organization  of  rgg,  l!),  2«J,  iiH  Jf. 
Oxyphils,  11, 

Pachytene,  252. 
Paedogenesis,  18,  52. 
Paracopulations/A'lle,  212,  225. 
Paramecium,  27. 
Paranucleus,  1(53. 
Paraplasm,  7. 
Parasitism,  191-192. 
Parasynapsis,  254. 
Parthenogenesis,   18,  47,   145,  240, 

265. 
Pea,  302,  303. 
Peden,  191. 
Pennaria,  87. 
Peripatus,  285,  288. 
Petromyzoru  33. 
Phalliisia,  233. 
Phosphatid,  8,  12. 
Phylloxera,  2G5  /. 
Physa,  18G,  225. 
Pig,  194. 
Planoccra,  157. 
Pla7iorbi.s\  ISO. 
Plasmodia,  artificial,  77-7S. 
Plasmosome,  5,  7,  102,  103,  213. 
Plastid,  5,  7. 
Plastochondria,  279. 
Plastokonta.  279. 
Plastosome,  7.  244,  275.  279. 
Polar  body,  47,  143-144. 
Polares  Plasma  (see  pole-plasm). 
Polarity,  19,  107.  \n,  179,  231/. 
Pole-cell,  110.  111.  117. 
Pole-disc,    1(>!>,   114.   117.    U2.   Jl!). 

225,  229.  2.3.-). 
Pole-plasm,   53  55.   228,   230,   235, 

294-295. 
Polistes,  i'i.'i. 
Polychoerus,  1 57 . 
PolyembryouN ,  145/.,  Kil. 
Polyp,  23! 
Polyphemus,  170/..  230. 


I  Polyspenny,  48.  115.  299. 

Porifcni,  iW)  Jf. 
I  Pol.ito  JMftl*'  {gee  L< ptinolartaK 

Prrl)l,i.sto«l«Tiiiic  tni<lfi.  114. 

Prt'«lrlfrniinali<»n.  2. 

Preformation.  2.  243. 

Prochrom(».some,  299. 

Progenninativr  c«-ll.  190.  197. 

Proiiiorplioltigy.  l!). 

I'riiplia.sc.  14. 

Protandry.  193. 

Protein.  8.  10. 

Protenor,  12.3.  258. 

Pr(»t(.gyny.  192-193. 

Protoplasm.  3   13. 

Protozoa,  1.  17.  25. 

Pteropod.  2<»;».  271. 

Pupa.  23. 

Pyrrhocori.t,  25(5. 

Rana,  32. 

Recessive  charjicter.  .304. 

He<lu(lion     of     clintmosomc*.     4.3, 

253. 
Regeneration.  79  SO.  297. 
RepnKlu<tion.  17   is. 
Hot  if  era.  ISO. 
Hhalxliti.-*,  207.  270. 
Ri(lifung.sropuIati«)nNkem.  1  U 

Sagitta.  179/..  195.  228. 

Salatiiandra,  134. 

SarciMJe,  3. 

SrorpcFua,  222. 

Sea  urchin.  210. 

Secondary  .sex  «-hnmrters.  ISO. 

Segregation  of  germ  <flls.  21*. 

Scif-copiiiation.  l!»2. 

Si'lf-fertilizjition.  192. 

S<Tt..li  cell.  .35.  129-1.33. 

Sex.  18.  189. 

Sc\  chromosonie.  255/. 

S'X  detennination,  271. 

Sol.  5.  0.  9. 

Sorite.  70,  79. 


346 


INDEX  OF  SUBJECTS 


Spermatogenesis,  42,  256  ff. 
Spermatogonia,  127. 
Spermatozoon,  19-22,  48. 
Spherule,  276. 
Spireme,  14,  15. 
Spongilla,  73. 
Spongioplasm,  4,  5. 
Sporulation,  17. 
Squash  bug,  256. 
Starfish,  6. 
Statoblast,  18. 
Statocyte,  70,  71,  73. 
Stem-cell,  175. 

Stone-fly,  hermaphroditic,  194. 
Synapsis,  44,  122,  250/.,  305. 
Synaptene,  251,  252. 
Synizesis,  43,  237,  251,  252. 

TcBTiia,  136,  137. 
Telophase,  15,  16. 
Telosynapsis,  254. 
Testis,  41. 
Tethya,  70,  76,  79. 


Tetrad,  44,  45. 

Tipulides,  107. 

Toad,  hermaphroditic,  207-208. 

Tokocyte,  71,  73,  79. 

Trophochromatin,  28. 

Unit  character,  304. 
Uterine  spindle,  157. 

Vacuole,  5,  8. 
Vegetative  pole,  20. 
Vertebrate,  32,  95-105,  212. 
Vitelline  membrane,  113,  114, 
Vitellophag,  114. 

X-chromosome,  255  ff.,  264,  299. 

Y-chromosome,  259  ff.,  264. 
Yolk,  in  germ  cells,  101,  224. 
Yolk  nucleus,  19,  226,  285. 

Zygosome,  251. 
Zygote,  1,  48. 


'T^HE  following  pages  contain  advertisements  of 
books  by  the  same  author  or  on  kindred  subjects 


An  Introduction  to  Zoology 

By 
ROBERT    W.    HEGNER,    Ph.D. 

Assistant  Professor  of  Zoology  in  the  University  of  Michigan 

A  TEXT-BOOK   INTENDED    FOR    THE    USE    OF    STUDENTS 
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interrelations  and  behavior)." 

"The  author  shows  a  keen  educative  instinct  ;  there  is  a  marked 
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of  experts,  who  have  read  particular  chapters,  has  secured  an  enviable 
freedom  from  mistakes.  There  is  a  very  useful  bibliography,  and  a  glos- 
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"The  book  is  cordially  recommended  as  giving  a  thorough  prepara- 
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"The  attempt  is  made  to  present  the  newer  zoology  to  the  beginner. 
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student  finds  himself  at  home  at  once  among  American  forms  and  Ameri- 
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general  biology  and  wish  to  gain  a  more  comprehensive  view  of  the  animal 
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on  the  market  in  several  important  respects:  (i)  the  animals  and  their 
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Within  the  past  decade  there  has  been  a  tendency  for  teachers  of  zoology 
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nent morphologist  recently  said,  "  Morphology  ...  is  no  longer  in  favor 
.  .  .  and  among  a  section  of  the  zoological  world  has  almost  fallen  into 
disgrace"  (Bourne).  The  study  of  the  form  and  structure  of  animals  is, 
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that  I  have  seen,  and  has  made  a  book  which  students  will  find  very  useful 
he  keeps  everything  in  such  entirely  simple  and  clear  outlines,  and  at  the  sainr 
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An  Outline  of 
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With  a  Description  of  some  of 
the  Phenomena  <which  it  explains 

By  MAYNARD    M.    METCALF,    Ph.D. 

Professor  of  Zoology,  Oberlin  College,  Oberlin,  Ohio 
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By  S.    HERBERT,  M.D.  (Vienna;,  M.k.C.S.  (Eng.  ), 

L.R.C.I'.  (Lond. ) 

The  First  Principles  of  Heredity 

Cloth,  /V9//.  ///..  *'    '.  5^.<v»  ttft 

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Heredity  and  related  questions,  without  at  the  same  time  assuming  any 
previous  knowledge  of  the  subject  on  the  reader's  j)art. 


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Though  there  are  hosts  of  books  dealing  with  Kvulutiun,  they  arc  cither 
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too  limited  in  their  trcatnunt  of  the  subject.  In  a  simple,  yet  scicntitic, 
manner,  the  author  here  presents  the  jjroblem  of  Kvolution  comprehen- 
sively in  all  its  aspects. 

CONTENTS 

Introduction  —  Evolution  in  General.  Palacontol<ijr>'. 

Section  I  —  Inorganic  Evolution.  Geographical  Dinribinion 

The  P: volution  of  Matter.  Part  II        Theories  of  Kvolution. 

Section  II  — Organic  Evolution  Section   III  —  Suj>«ior  •  •■  •    i\   1. .•..-. 

Part  i  -  The  Facts  of  Evolution.  Social  I 

Morphology.  Conclusion  —  The  Formula  of  Evoluiim. 

Embryology.  The  Philowphy  of  Chanec 

Classification. 


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